How Sustainable is Stored Sunlight?

This is a text only version. Find the illustrated article at Low-tech Magazine.

One of the constraints of solar power is that it is not always available: it is dependent on daylight hours and clear skies. In order to fill these gaps, a storage solution or a backup infrastructure of fossil fuel power plants is required — a factor that is often ignored when scientists investigate the sustainability of PV systems. Whether or not to include storage is no longer just an academic question. Driven by better battery technology and the disincentivization of grid-connected solar panels, off-grid solar is about to make a comeback. How sustainable is a solar PV system if energy storage is taken into account?

In the previous article, we have seen that many life cycle analyses (LCAs) of solar PV systems have a positive bias. Most LCAs base their studies on the manufacturing of solar cells in Europe or the USA. However, most panels are now produced in China, where the electric grid is about twice as carbon-intensive and about 50% less energy efficient. [1] Likewise, most LCAs investigate solar PV systems in regions with a solar insolation typical of the Mediterranean region, while the majority of solar panels have been installed in places with only half as much sunshine.

As a consequence, the embodied greenhouse gas emissions of a kWh of electricity generated by solar PV is two to four times higher than most LCAs indicate. Instead of the oft-cited 30-50 grams of CO2-equivalents per kilowatt-hour of generated electricity (gCO2e/kWh), we calculated that the typical solar PV system installed between 2008 and 2014 produces close to 120 gCO2e/kWh. This makes solar PV only four times less carbon-intensive than conventional grid electricity in most western countries.

However, even this result is overly optimistic. In the previous article, we didn’t take into account “one of the potentially largest missing components” [2] of the usual life cycle analysis of PV systems: the embodied energy of the infrastructure that deals with the intermittency of solar power. Solar insolation varies throughout the day and throughout the season, and of course solar energy is not available after sunset.

Off-grid Solar Power is Back

Until the end of the 1990s, most solar installations were off-grid systems. Excess power during the day was stored in an on-site bank of lead-acid batteries for use during the night and on cloudy days. Today, almost all solar systems are grid-connected. These installations use the grid as if it was a battery, “storing” excess energy during the day for use at night and on cloudy days.

Obviously, this strategy requires a backup of fossil fuel or nuclear power plants that step in when the supply of solar energy is low or nonexistent. To make a fair comparison with conventional grid electricity, including electricity generated by biomass, this “hidden” part of the solar PV system should also be taken into account. However, every single life cycle analyse of a solar PV ignores it. [3, 2].

Until now, whether or not to include backup power or storage systems was mainly an academic question. This might change soon, because off-grid solar is about to make a comeback. Several manufacturers have presented storage systems based on lithium-ion batteries, the technology that also powers our gadgets and electric cars. [4, 5, 6, 7] Lithium-ion batteries are a superior technology compared to the lead-acid batteries commonly used in off-grid solar PV systems: they last longer, are more compact, more efficient, easier to maintain, and comparatively more sustainable.

Lithium-ion batteries are more expensive than lead-acid batteries, but Morgan Stanley’s 2014 report on solar energy predicts that the price of storage will come down to $125-$150 per kWh by 2020. [8] According to the report, this would make solar PV plus battery storage commercially viable in some European countries (Germany, Italy, Portugal, Spain) and across most of the United States. Morgan Stanley expects a lot from electric vehicle manufacturer Tesla, who announced a home storage system for solar power a few days ago (costing $350 per kWh). [9] Tesla is building a factory in Arizona that will produce as many lithium-ion batteries as there are currently produced by all manufacturers in the world, introducing economies of scale that can push costs further down.

Other factors also come into play when it comes to home storage for PV power. Solar panels have become so much cheaper in recent years that government subsidies and tax credits for grid-connected systems have come under pressure. In many countries, owners of a grid-connected solar PV system have received a fixed price for the surplus electricity they provide to the grid, without having to pay fixed grid rates. These so-called “net metering rules” or “feed-in rates” were recently abolished in several European countries, and are now under pressure in some US states. In its report, Morgan Stanley predicts that, in the coming years, net metering rules and solar tax credits will disappear altogether. [8]

Utility companies are fighting the incentivisation of PV power succesfully with the argument that solar customers make use of the grid but don’t pay for it, raising the costs for non-solar customers. [10] The irony is that the disincentivization of grid-connected solar panels makes off-grid systems more attractive, and that utilities might be chasing away their customers. If a grid-connected solar customer has to pay fixed grid fees and doesn’t receive a good price for his or her excess power, it might become more financially savvy to install a bank of batteries. The more customers do this, the higher the costs will become for the remaining consumers, encouraging more people to adopt off-grid systems. [11]

Lead-Acid Battery Storage

Being totally independent of the grid might sound attractive to many, but how sustainable is a solar PV system when battery storage is taken into account? Because a life cycle analysis of an off-grid solar system with lithium-ion batteries has not yet been done, we made one ourselves, based on some LCAs of stand-alone solar PV systems with lead-acid battery storage.

One of the most complete studies to date is a 2009 LCA of a 4.2 kW off-grid system in Murcia, Spain. The 35 m2 PV solar array is mounted on a building rooftop and supplies a programmed lighting system with a daily constant load pattern of 13.8 kWh. The solar panels are connected to 24 open lead-acid batteries with a storage capacity of 110.4 kWh, offering three days of autonomy. [12] The study found an energy payback time of 9.08 years and specific greenhouse gas emissions of 131 gCO2e/kWh, which makes the system twice as energy efficient and 2.5 times less carbon-intensive than conventional grid electricity in Spain (337 gCO2/kWh). Manufacturing the batteries accounts for 45% of the embodied CO2, and 49% of the life cycle energy use of the solar system.

This doesn’t sound too bad, but unfortunately the researchers made some pretty optimistic assumptions. First of all, the results are valid for a solar insolation of 1,932 kWh/m2/yr — Murcia is one of the sunniest places in Spain. At lower solar insolation, more solar panels would be needed to produce as much electricity, so the embodied energy of the total system will increase. [13]. If we assume a solar insolation of 1,700 kWh/m2/yr, the average in Southern Europe, GHG emissions would increase to 139 gCO2e/kWh. If we assume a solar insolation of 1,000 kWh/m2/yr, the average in Germany, emissions amount to 174 gCO2/kWh.

Battery Lifespan

Secondly, the researchers assume the lifespan of the lead-acid batteries to be 10 years. For the solar panels, they assume a lifetime of 20 years, which means that they included double the amount of batteries in the life cycle analysis. A lifespan of ten years is very optimistic for a lead-acid battery — a fact that the scientists admit. [12] Most other LCA’s looking at off-grid systems assume a battery life of 3 or 5 years [14, 15]. However, the lifetime of a lead-acid battery depends strongly on use and maintenance. Because of the low load of the system under discussion, a battery lifespan of 10 years is not completely unrealistic.

On the other hand, if the batteries are used for higher loads — for example, in a common household — their lifetime would shorten considerably. Because almost 50% of embodied CO2 and life cycle energy use of a PV solar system is due to the batteries alone, the expected lifespan of the 2.4 ton battery pack has a profound effect on the sustainability of the system.

If we assume a battery lifespan of 5 instead of 10 years, and keep the other parameters the same, the GHG emissions increase to 198 and 233 gCO2e/kWh for a solar insolation of 1,700 and 1,000 kWh/m2/yr, respectively. In grid-connected solar PV systems, assuming a longer life expectancy for the solar panels improves the sustainability of the system: the embodied energy and CO2 can be spread over a longer period of time. With off-grid systems, this effect is countered by the need for one or more replacements of the batteries.

If we increase the life expectancy of the solar panels from 20 to 30 years, and keep the battery lifespan at 10 years, CO2e emissions per kWh remain more or less the same. However, if we assume a battery lifespan of only 5 years and extend the lifespan of the solar panels to 30 years, GHG emissions would increase to 206 gCO2e/kWh for a solar insolation of 1,700 kWh/m2/yr, and decrease to 232 gCO2e/kWh for a solar insolation of 1,000 kWh/m2/yr.

Made in China

Thirdly, the researchers assume that all components — PV cells, batteries, electronics — are made in Spain, while we have seen in the previous article that manufacturing of solar PV systems has moved to China. Spain’s electricity grid is 2.7 times less carbon-intensive (337 gCO2/kWh) than China’s electric infrastructure (900 gCO2e/kWh), which means that the GHG emissions of all components of our system can be multiplied by 2.7. This results in specific carbon emissions of 353 and 471 gCO2e/kWh for a solar insolation of 1,700 and 1,000 kWh/m2/yr, respectively, which is higher than the carbon-intensity of the Spanish grid. Considering a battery lifespan of 5 instead of 10 years, emissions would rise to 513 and 631 gCO2e/kWh for a solar insolation of 1,700 and 1,000 kWh/m2/yr, respectively.

Although there are some assumptions by the researchers that are less optimistic — such as a battery recycling rate of only 50% instead of the more commonly assumed +90% — it’s obvious that an off-grid system with lead-acid batteries is not sustainable, and definitely not when the components are manufactured in China. That doesn’t make off-grid solar with lead-acid batteries pointless: compared to a diesel generator, a solar PV system with lead-acid batteries is often the better choice, which makes it a good solution for remote areas without access to the power grid. As an alternative for the centralized electricity infrastructure in western countries, however, it makes little sense.

Lithium-ion Battery Storage System

When we replace the lead-acid batteries by lithium-ion batteries, the sustainability of a stand-alone solar PV system improves considerably. At first glance this may seem counter-productive, because it takes more energy to produce 1 kWh of lithium-ion battery storage than it takes to manufacure 1 kWh of lead-acid battery storage. According to the latest LCA’s, aimed at electric vehicle storage, the making of a lithium-ion battery requires between 1.4 and 1.87 MJ/wh, [16, 17, 18] while the energy requirements for the manufacture of a lead-acid battery are between 0.87 and 1.19 MJ/Wh. [18, 12]

Despite this, the higher overall performance of the lithium-ion battery means that considerably less storage is required. For a prolonged lifetime, lead-acid batteries demand a limited “Depth of Discharge” (DoD). If a lead-acid battery is fully discharged (DoD of 100%) its lifespan becomes very short (300 to 800 cycles, or roughly one to two years, depending on battery chemistry). The lifespan increases to between 400 and 1,000 cycles (1-3 years, assuming 365 cycles per year) at a DoD of 80%, and to between 900 and 2,000 cycles (2.5-5.5 years) at a DoD of 33%. [18]. This means that, in order to get a decent lifespan, a lead-acid battery system should be oversized. For example, three times more battery capacity is needed at a DoD of 33%, because two thirds of the battery capacity cannot be used.

Although the lifespan of a lithium-ion battery also decreases when the depth of discharge increases, this effect is less pronounced than with its lead-acid counterpart. A lithium-ion battery lasts 3,000 to 5,000 cycles (8-14 years) at a DoD of 100%, 5,000 to 7,000 cycles (14-19 years) at a DoD of 80%, and 7,000 to 10,000 cycles (19-27 years) at a DoD of 33%. [18] As a consequence, lithium-ion storage usually has a DoD of 80%, while lead-acid storage usually has a DoD of 33 or 50%. In the LCA of the Spanish off-grid system discussed above, the assumption of three days of autonomy implies that 41 kWh of storage is required (3 x 13.8 kWh per day). Because the DoD is 33%, total storage capacity should be multiplied by three, which results in 123 kWh of batteries. If we would replace these by lithium-ion batteries with a DoD of 80%, only 50 kWh of storage is needed, or 2.5 times less.

6 x Less Batteries Needed

For utmost accuracy, we should mention that the lifespan of a battery isn’t necessarily limited by the cycle life. When batteries are used in applications with shallow cycling, their service life will normally be limited by float life. In this case, the difference between lead-acid and lithium-ion is less pronounced: at no-cycling (float charge), lithium-ion lasts 14-16 years and lead-acid 8-12 years. Battery life will be limited by either the life cycle or the float service life, depending on which condition will be achieved first. [18] Nevertheless, if we focus on off-grid systems for households, the assumption of deep daily cycling better reflects reality, although there will be periods of float charge, for example during holidays.

If we also factor in the lifespan of the batteries, the advantage of lithium-ion becomes even larger. Assuming a lifespan of 20 years for the solar PV system and a DoD of 80%, the lithium-ion batteries will last as long as the PV panels. On the other hand, the lead-acid batteries have to be replaced at least 2-4 times over a period of 20 years. This further widens the gap in energy use for manufacturing when comparing lead-acid and lithium-ion batteries. [18] In the original LCA, a total storage capacity of about 240 kWh is needed over a lifespan of 20 years. On the other hand, the cycle life of the lithium-ion battery is 19-27 years, meaning that no replacement may be needed. Consequently, the total storage capacity to be manufactured over the complete lifetime of the system is 6 times lower for lithium-ion than for lead-acid. [19]

If we take the most optimistic values for energy during manufacturing, being 0.87 MJ/Wh for lead-acid and 1.4 MJ/Wh for lithium-ion, and multiply them by total battery capacity over a lifetime of 20 years (248,000 Wh for lead-acid and 42,000 Wh for lithium-ion), this results in an embodied energy of 60 MWh for lead-acid (the value in the original LCA) and only 16.5 MWh for lithium. In conclusion, energy requirements for the manufacturing of the batteries is 3.6 times lower for lithium-ion than for lead-acid.

Another advantage of lithium-ion batteries is that they have a higher efficiency than lead-acid batteries: 85-95% for lithium-ion, compared to 70-85% for lead-acid. Because losses in the battery must be compensated with higher energy input, a higher battery efficiency results in a smaller PV array, lowering the energy requirements to manufacture the solar cells. In the original LCA, 4.2 kW of solar panels (35 m2) are needed to produce 13.8 kWh per day. If we assume the lead-acid batteries to be 77% efficient, and the lithium-ion batteries to be 90% efficient, the choice for lithium-ion would resize the solar PV array from 4.2 kW to 3.55 kW. We now have all the data to calculate the greenhouse gas emissions per kWh of electricity produced by an off-grid solar PV system using lithium-ion batteries.

GHG Emissions of the Off-grid System with Lithium-ion Batteries

In the original LCA, the batteries and the solar panels (including frames and supports) account for 59 and 62 gCO2e/kWh, respectively. The rest of the components add another 10 gCO2e/kWh, resulting in a total of 131 gCO2e/kWh. If we switch to lithium-ion battery storage, the greenhouse gas emissions for the batteries come down from 59 to 20 gCO2e/kWh. Because of the higher efficiency of the lithium-ion batteries, the greenhouse gas emissions for the solar panels come down from 62 to 55 gCO2e/kWh. This brings the total greenhouse gas emissions of the off-grid system using lithium-ion batteries to 85 gCO2e/kWh, compared to 131 gCO2e/kWh for a similar system with lead-acid storage.

While this result is an improvement, it’s dependent on the assumptions of the researchers; most notably, a solar insolation of 1,932 kWh/m2/yr, and that all manufacturing of components occurs in Spain. If we adjust the value for a solar insolation of 1,700 kWh/m2/yr in order to compare with the other results, total GHG emissions become 92.5 gCO2e/kWh (assuming battery capacity remains the same). If we correct for a solar insolation of 1,000 kWh/m2/yr, the average in Germany, GHG emissions become 123.5 gCO2e/kWh. Furthermore, if we assume that the solar panels (but not the batteries or the other components) are manufactured in China, which is most likely the case, GHG emissions rise to 155 and 217 gCO2e/kWh for a solar insolation of 1,700 and 1,000 kWh/m2/yr, respectively.

In conclusion, lithium-ion battery storage makes off-grid solar PV less carbon-intensive than conventional grid electricity in most western countries, even if the manufacturing of solar panels in China is taken into account. However, the advantage is rather small, which effects the speed at which solar PV systems can be deployed in a sustainable way. In the previous article, we have seen that the energy and CO2 savings made by the cumulative installed capacity of solar PV systems are cancelled out to some extent by the energy use and CO2 emissions from the production of new installed capacity. For the deployment of solar systems to grow while remaining net greenhouse gas mitigators, they must grow at a rate slower than the inverse of their CO2 payback time. [20, 21, 22]

For solar panels manufactured in China and installed in countries like Germany, the maximum sustainable growth rate is only 16-23% (depending on solar insolation), roughly 3 times lower than the actual annual growth of the industry between 2008 and 2014. If we also take lithium-ion battery storage into account, the maximum sustainable growth rate comes down to 4-14%. In other words, including energy storage further limits the maximum sustainable growth rate of the solar PV industry.

On the other hand, if we would produce solar panels in countries with very clean electricity grids (France, Canada, etc.) and install them in countries with carbon-intensive grids and high solar insolation (China, Australia, etc.), even off-grid systems with lithium-ion batteries would have GHG emissions of only 26-29 gCO2/kWh, which would allow solar PV to grow sustainably by almost 60% per year. This result is remarkable and shows the importance of location if we want solar PV to be a solution instead of a problem. Of course, whether or not there’s enough lithium available to deploy battery storage on a large scale, is another question.

Battery Production Powered by Renewable Energy?

Another way to improve the sustainability of battery storage is to produce the batteries using renewable energy. For example, Tesla announced that its “GigaFactory”, which will produce lithium-ion batteries for vehicles and home storage, will be powered by renewable energy. [23, 24] To support their claim, Tesla published an illustration of the factory with the roof covered in solar panels and a few dozen windmills in the distance.

However, the final manufacturing process in the factory consumes only a small portion of the total energy cost of the entire production cycle — much more energy is used during material extraction (mining). It’s stated that the GigaFactory will produce 50 GWh of battery capacity per year by 2020. Because the making of 1 kWh of lithium-ion battery storage requires 400 kWh of energy [16, 17, 18], producing 50 GWh of batteries would require 20,000 GWh of energy per year.

If we assume an average solar insolation of 2,000 kWh/m2/yr and a solar PV efficiency of 15%, one m2 of solar panels would generate at most 295 kWh per year. This means that it would take 6,800 hectares (ha) of solar panels to run the complete production process of the batteries on solar power, while the solar panels on the roof cover an area of only 1 to 40 ha (there is some controversy over the actual surface area of the factory under construction). Tesla’s claim, though potentially factually accurate, is an obvious example of greenwashing — and everyone seems to buy it.

There are other ways to improve the sustainability of solar PV when storage is taken into account. Most of these solutions require that solar systems remain connected to the grid, even if they have a (more limited) local storage system. In this scenario, chemical batteries could help to balance the grid system, acting as peak-shaving and load-shifting devices. The electric grid has to be sized to meet peak demand, and battery storage could mean that less power plants are needed for that. Decentralized, grid-connected energy storage could also increase the share of renewables that the electricity infrastructure can handle. Of course, this “smart grid” approach should also be subjected to a life cycle analysis, including all electronic components.

Kris De Decker (edited by Jenna Collett)

EDIT: the paragraph about Tesla’s GigaFactory was rewritten to reflect the fact that most energy is consumed during material extraction.

Sources & Notes:

[1] Domestic and overseas manufacturing scenarios of silicon-based photovoltaics: life cycle energy and environmental comparative analysis. Dajun Yue, Fengqi You, Seth B. Darling, in Solar Energy, May 2014

[2] Energy Payback for Energy Systems Ensembles During Growth (PDF), Timothy Gutowski, Stanley Gershwin and Tonio Bounassisi, IEEE, International Symposium on Sustainable Systems and Technologies, Washington D.C., May 16-19, 2010

[3] “Current State of Development of Electricity-Generating Technologies: A Literature Review”, Manfred Lenzen, Energies, Volume 3, Issue 3, 2010.

[4] “Storage is the new solar: will batteries and PV create an unstoppable hybrid force?”, Stephen Lacey, Greentechmedia, 2015

[5] “Report: Solar Paired with Storage is a ‘Real, Near and Present’ Threat to Utilities”, Stephen Lacey, greentechmedia, 2014

[6] “Australia to pilot new power plan”, Gregg Borschmann, ABC, May 2014

[7] “SolarCity Launches Energy Storage for Business Using Tesla Battery Packs”, Eric Wesoff, greentechmedia, December 2013

[8] “Solar Power & Energy Storage: Policy Factors vs. Improving Economics” (PDF), Morgan Stanley Blue Paper, July 28, 2014

[9] “Tesla announces home battery system”, Slashdot, May 1, 2015

[10] “Utilities wage campaign against rooftop solar”, Joby Warrick, The Washington Post, March 2015

[11] “Disruptive Challenges: Financial Implications and Strategic Responses to a Changing Retail Electric Business” (PDF), Peter Kind, Energy Infrastructure Advocates, Edison Electric Institute, January 2013

[12] “Life cycle assessment study of a 4.2kWp stand-alone photovoltaic system”, R. García, in “solar energy”, september 2009.

[13] You could argue that you also need more battery storage, because there is a bigger chance of cloudy days. However, we assume battery capacity to remain the same.

[14] “Optimal Sizing and Life Cycle Assessment of Residential Photovoltaic Energy Systems With Battery Storage”, A. Celik, in “Progress in Photovoltaics: Research and Applications”, 2008.

[15] “Energy pay-back time of photovoltaic energy systems: present status and prospects”, E.A. Alsema, in “Proceedings of the 2nd World Conference and Exhibition on photovoltaics solar energy conversion”, July 1998.

[16] “Towards greener and more sustainable batteries for electrical energy storage”, D. Larcher and J.M. Tarascon, Nature Chemistry, November 2014

[17] “Application of Life-Cycle Assessment to Nanoscale Technology: Lithium-ion Batteries for Electric Vehicles” (PDF), Environmental Protection Agency (EPA), 2013

[18] “Energy Analysis of Batteries in Photovoltaic systems. Part one (Performance and energy requirements)” (PDF) and “Part two (Energy Return Factors and Overall Battery Efficiencies)” (PDF). Energy Conversion and Management 46, 2005.

[19] The lifespan of the lithium-ion battery will probably be closer to 14-16 years (float charge lifespan) because of the shallow cycling assumption in the original LCA. However, since the assumed lifespan of 10 years for the lead-acid batteries is very optimistic, and because deep cycling is more common for household off-grid systems, we assume that no replacement of lithium-ion batteries is needed.

[20] “The climate change mitigation potential of the solar PV industry: a life cycle perspective”, Greg Briner, 2009

[21] “Optimizing Greenhouse Gas Mitigation Strategies to Suppress Energy Cannibalism” (PDF). J.M. Pearce. 2nd Climate Change Technology Conference, May 12-15, 2009, Hamilton, Ontario, Canada.

[22] “Towards Real Energy Economics: Energy Policy Driven by Life-Cycle Carbon Emission”, R. Kenny, C. Law, J.M. Pearce, Energy Policy 38, pp. 1969-1978, 2010

[23] “Construction of Tesla’s $5B solar-powered Gigafactory in Nevada is progressing nicely”, Michael Graham Richard, Treehugger 2014

[24] “Tesla’s $5bn Gigafactory looks even cooler than expected, will create 22,000 jobs”, Michael Graham Richard, Treehugger 2015

Posted on May 03, 2015 at 04:18 PM in Batteries, Cover story, Energy, Solar energy | Permalink



“Tesla’s claim is an obvious example of greenwashing — and everyone seems to buy it.”

They never said that electricity would be produced 100% on-site. The Gigafactory is located next to some of the best onshore wind resources in the nation, and the image posted above even includes windmills.

A company can buy renewable energy from the market.

Posted by: Eric W. | May 03, 2015 at 07:11 PM


@ Eric

Even then, aren’t the solar panels on the roof a textbook example of greenwashing? The image gives the impression that a roof full of solar panels and some windmills in the distance suffice to make the factory work.

And I guess those wind resources nearby are now supplying electricity for other uses. So new windmills will have to be built.

Posted by: Kris De Decker | May 03, 2015 at 07:19 PM


How do these numbers compare to Nikel/Iron batteries with expected lifespans more conveniently measured in decades rather than years?

Posted by: Charles | May 03, 2015 at 07:26 PM


@ Charles

Unfortunately, answering that question would require a whole new calculation. Considering how much time it took to make this one, I won’t be able to give you an answer anytime soon…

Posted by: Kris De Decker | May 03, 2015 at 08:15 PM


This is a very useful article. One small correction: Tesla’s “gigafactory” is in northern Nevada near Reno, not in Arizona, so the solar insolation would be less.

Another research article I didn’t see in your references that looks at solar and storage from an energy cost angle (but not emissions) is Carbajales-Dale et al, 2014, at It comes to a similar conclusion: At 72 hours storage, most PV systems with storage cannot pay back their energy ‘debt’; wind, however, because of a more favorable EROI, can ‘afford’ more storage.

Posted by: David F | May 03, 2015 at 09:05 PM


@ David

Thanks for the correction, I will make that calculation again. And thanks for the link, I didn’t know that paper. Indeed, wind does much better than solar. It’s remarkable that solar PV grows too fast to be sustainable, while wind remains far below its maximum sustainable growth rate.

And although many will read my article as a criticism of PV solar energy, for me the most surprising fact was that even an off-grid solar system can have GHG emissions below 30 gCO2e/kWh if you manufacture and install it in the right locations. It’s not that renewable energy is fundamentally flawed, we just have to reconsider our priorities.

PS: I corrected your link, it didn’t work.

Posted by: Kris De Decker | May 04, 2015 at 01:33 AM


Regarding the claims about Tesla’s factory and PV.

Tesla have claimed that the factory will run on 100% renewable energy.

The calculation which claims to show green-washing includes all of the embedded energy in the materials used to assemble battery packs. For example the battery packs will contain metals, however embedded energy for these metals will be consumed at a smelter not at the gigafactory.

So I believe the calculation in this article is misleading, and that Tesla’s claim may well be true.

Posted by: Greg | May 04, 2015 at 10:01 AM


The size is of the proposed factory is about 1000m x 400m giving a roof area of 40 hectares not 1 hectare

Posted by: Daron Robinson | May 04, 2015 at 10:07 AM


Great work!

A life cycle analysis of a grid connected system with only night time storage would be very interesting.

Posted by: Martin Lindh | May 04, 2015 at 10:13 AM


Some interesting thoughts. I certainly welcome it when people consider the emission impacts of renewable energy and you’re probably right that there is a positive bias on LCAs there. However I’d assume that you can find similar positive biases on LCAs for other energy forms as well, not sure which ones are worse (e.g. there’s been a lot of debate about methane emissions from natural gas in recent years that were systematically underestimated in the past).

I see the off-grid solar/battery solutions as a second best option. I don’t really like them, but realistically we’ll get them and they’re better than staying with the status quo. The alternative – intelligent options using the grid – have one huge disadvantage: You need the grid operator to cooporate.

To put it mildly, most grid operators aren’t exactly on the forefront of renewable innovation. They’re often tied (or often just are) to traditional utilities that have little interest in change.

For a solution with medium and large scale storage and more intelligent grids (things like demand side management) you’d either need grid operators support (unlikely) or a very strong political regulation towards renewables (also unlikely in most countries, fossil/nuclear lobbies are strong). I live in Germany and we have relatively good political support for renewables, but even here politics is switching back and forth between supporting renewables and slowing the development down to support the old utilities. A couple of years ago there were several plans for new large scale storages (mostly pumped water, some experimental like underground pumped water or air pressure), most of them are on hold because there is little incentive right now to build them

Posted by: Hanno | May 04, 2015 at 10:22 AM


I appreciate this article but I’m skeptical of the conclusion. My background is electrical engineering and I’ve done some basic PV calculations in the past, but you should get feedback from a power engineer who has experience in this field.

This article on came up with different numbers.

Musks companies are filled with engineers, the finest in the world, each sworn to an oath of ethics. Musks fanbase is filled with millions more engineers. If it was as unfeasible as your article suggests, I would expect an army of engineers talking about this online.

I’m not saying your wrong. It’s great to be skeptical. But I’m skeptical of your conclusion about the Gigafactory.

Posted by: Tim Perry | May 04, 2015 at 11:16 AM


Some points :

– nuclear can’t step in for day/night variation, the ramp up of nuclear plants longer than a day, that’s why nuclear is baseline in heavily nuclear countries like France
– there is a renewable that can ramp up instantly : hydroelectric
– 100% renewable without storage is theoretically possible, it will take a global grid, and then it is always sunny or windy somewhere…

Posted by: Bruno | May 04, 2015 at 12:24 PM
Nick Nolan


20,000 GWh (20 000 000 MWh) per year is obviously incorrect estimate. It’s equal to 2283 MW power consumption 24/7 (two AP1000 nuclear reactors dedicated for one factory).

Source you are using gives total energy cost of battery. The majority of the total energy usage is in material production, not battery production.

Tesla recycles all lithium in batteries, so eventually even the total energy usage will be lower. The amount of lithium they use is enormous. The whole endeavor would be financially unsound without recycling.

Posted by: Nick Nolan | May 04, 2015 at 01:28 PM


Great article. I wonder if it’S possible that the 400 kWh needed to produce a lithium ion battery will be reduced by scaling to mass production in the gigafactory? Or is this just the base energy required to bring the chemistry together in the battery?

Posted by: Dan | May 04, 2015 at 02:38 PM


Did you go from 68,000sqm to 68sqkm? If so, you are off by a thousand, and the area needed is 0.68sqkm (68ha). Also, where do you get 1ha for the roof from? Qick googling gets me in the ballbark of 1km long and 450 meters wide, which is 45ha. So half from solar on roof and rest from windmills outside doesnt seem too bad to me

Posted by: Henrik | May 04, 2015 at 03:11 PM


Sorry my bad about the sqm calculation, made and error going from Gigawatt to Kilowatt I think.

Posted by: Henrik | May 04, 2015 at 03:26 PM


” Because the manufacture of 1 kWh of lithium-ion battery storage requires 400 kWh of energy, the factory would require 20,000 GWh of electricity per year to manufacture all these batteries. ”

Can you provide any proof of this statement ?

I suspect that your figures are a factor 100x too high (1kWh lithium storage requires only 4kWh of energy instead of 400kWh as you say).

As such your 68 square kilometers or solar PV panels required to power this factory, would be dead wrong, and Tesla would be dead right ….

See page 32-> 35kWh needed to produce one ingot of 1 kg of pure Li.

See page 33 -> Through our calculation, it means only 0.017k
Wh electricity is consumed to produce 1 Ah Li battery. It is only 3% of the energy, which is used in battery production (0.54kWh/1Ah battery). Lithium battery industry, only 4.7kg lithium metal
would be used in a 30kWh size Li-battery on average according to Argonne National Laboratory’s

Page 40 -> Lifecycle analysis of lithium iron battery by
Mats Zackrisson and Lars Avellán in 2010 claims that the total energy consumption corresponds to 11.7 kWh electricity and 8.8 kWh of thermal energy from natural gas per kg lithium-ion battery

Posted by: Alain | May 04, 2015 at 05:18 PM


” Remarkably, Tesla shows an illustration of the factory with solar panels on the roof. Knowing that the factory will occupy a surface of 1 ha, while 6,800 ha of solar panels is required to run it on renewable energy, Tesla’s claim is an obvious example of greenwashing — and everyone seems to buy it. ”

You are wrong, the factory sits on a 980 acre site according to Tesla’s weblink, that is 4 square kilometers, so plenty of space (400 hectare) available for the 68 hectares of PV solar panels that are neede to cover all the plants energy consumption. And Tesla will also build a 140MW wind turbine park, providing unknown GWh in extra power to make the plant a zero net energy facility, as they correctly stated.

Posted by: Alain | May 04, 2015 at 05:44 PM


Great set of articles. Made me rethink solar PV. If/when I buy it will be panels made somewhere besides Asia.

The Tesla factory is in AZ. I checked and AZ gets the majority of its power from coal (39%) followed by nuclear (28%) and renewable (10% – mostly hydro). Not sure where the rest comes from. Crazy that so little is from solar despite having the strongest sunlight in the US, Canada or Europe. Obviously there is an imbalance and opportunity.

Posted by: Steve | May 04, 2015 at 07:56 PM


Tesla is in Nevada (correction)

“90% of the energy Nevada consumes comes from outside the state” (

Posted by: Steve | May 04, 2015 at 08:01 PM


You say: “the manufacture of 1 kWh of lithium-ion battery storage requires 400 kWh of energy, the factory would require 20,000 GWh of electricity per year to manufacture all these batteries.”

This is wildly off. The only way you can plausibly get a figure of 400 kWh is to be looking at the total energy cost of the entire production process, from material extraction all the way through to final assembly (and even then 400 kWh seems too high, based on the sources you’ve cited). The final manufacturing phase – what will be done in the Tesla Gigafactory – consumes only a small portion of the total energy cost of the entire production cycle.

You may be right that the Gigafactory will not be able to meet its energy requirements from renewables, but not based on these figures. Working in your favour is the fact that the factory will in fact be in Nevada, where solar insolation is lower than in Arizona. Working against you is the fact that all the energy cost numbers available to work with are almost certainly wrong: they don’t account for the somewhat uncommon battery chemistry that Tesla use, or for the new economies of scale that might be achieved by a factory of this size (although the scope for this is limited by the simple fact that 75% of the total energy cost of production is wrapped up in material extraction).

This is a really important question, one that it’s really important to get right.

Posted by: William | May 04, 2015 at 08:29 PM
Spiffy Solar


Many just assume that given the right price, batteries will become a major answer to our power generation woes. That they will take over the market and perhaps the utilities as well.

Making the right decisions on when to utilize batteries sounds crucial to their usefulness.

Another important factor comes to light, upon close consideration. That is, the technology to maintain and use batteries to their full effectiveness.

Thanks for the illuminating article.

Posted by: Spiffy Solar | May 04, 2015 at 08:34 PM


“Morgan Stanley expects a lot from electric vehicle manufacturer Tesla, who announced a home storage system for solar power a few days ago (costing $350 per kWh”

You are correct in that the home solution is priced at $350 a kWh, but you should also note that Tesla is announced to sell their 100KW commercial battery storage solution for $25,000 – which is $250 per KW. That’s a pretty good deal for storage costs.

Posted by: Steve | May 04, 2015 at 09:08 PM
Thom the Turtle


I really enjoyed the article, but I wished it would have addressed behavioral changes that arise when power is finite, unlike the grid delivering functionally infinite amounts at all times of the day. Just as people adapt to a cell phone that drops to 20%, they will do the same in a house run on batteries powered by sunlight.

FWIW, I currently live entirely off-grid, although I do own a generator that I use about twice a month for construction projects. I helped install a small, off-grid solar system on Sunday and the issues in this article were discussed.

Posted by: Thom the Turtle | May 04, 2015 at 10:19 PM
Kris De Decker


Sorry for the delay in publishing the comments, I had no internet access all day.

Many of the comments address the last paragraph, which accuses Tesla of greenwashing when it states that it will power its Gigafactory by renewable energy.

I should partly correct that statement. As some of you wrote, the 20,000 GWH energy use per year (the number is correct) concerns the total energy cost of all batteries produced annually in the factory, including material production. The mining of raw materials doesn’t happen in the factory itself, so in that sense my claim is incorrect, and Tesla’s claim might be right.

However, Tesla’s claim remains greenwashing, even more so. The image and the sales talk give the impression that batteries will be produced by renewable energy, while in truth battery manufacturing in the factory itself consumes only a small share of the total energy cost of the complete production process.

Or, as one commenter on Hackernews phrased it: “Why crow over renewable energy when your use of energy is irrelevant relative to your suppliers”.

Tesla fits itself a green image by running its factory on solar and wind, but doesn’t say a word about the much larger energy use of material production.

I have edited the paragraph to make the point clear.

Posted by: Kris De Decker | May 05, 2015 at 12:09 AM
Michael Dowling


What most concerns me is getting away from centralized power generation,with it’s high voltage power transformers,for which there are,in many cases, no spares.I’ve read these things are hand built,weigh hundreds of tons,and take literally years to build and transport.If enough of them were destroyed due to a severe geomagnetic storm or a coordinated terrorist attack,grid power could be down for months or even years.Source:

Posted by: Michael Dowling | May 05, 2015 at 04:57 AM


” I should partly correct that statement. As some of you wrote, the 20,000 GWH energy use per year (the number is correct) concerns the total energy cost of all batteries produced annually in the factory, including material production. The mining of raw materials doesn’t happen in the factory itself, so in that sense my claim is incorrect, and Tesla’s claim might be right.”

Can you prove that statement of 20 TWh needed in energy ?

I for one does not buy it, without proof.

Critizising is very easy, when you don’t have any figures at hand to state your case. That is what politicians do to sell their boondoggles to gullible voters. Your readers are often engineers, and they need facts, not baseless statements.

So where did you get this 20 TWh energy needed to make those batteries, starting from material mining ?

If you can’t, then at least have the technical decency to not mention wrong figures based on zero facts, and have the intellectual decency to not launch baseless attacks on companies doing something that you aren’t doing…

Posted by: alain | May 05, 2015 at 01:09 PM


” However, the final manufacturing process in the factory consumes only a small portion of the total energy cost of the entire production cycle — much more energy is used during material extraction (mining). It’s stated that the GigaFactory will produce 50 GWh of battery capacity per year by 2020. Because the making of 1 kWh of lithium-ion battery storage requires 400 kWh of energy [16, 17, 18], producing 50 GWh of batteries would require 20,000 GWh of energy per year.

[16] “Towards greener and more sustainable batteries for electrical energy storage”, D. Larcher and J.M. Tarascon, Nature Chemistry, November 2014

[17] “Application of Life-Cycle Assessment to Nanoscale Technology: Lithium-ion Batteries for Electric Vehicles” (PDF), Environmental Protection Agency (EPA), 2013

[18] “Energy Analysis of Batteries in Photovoltaic systems. Part one (Performance and energy requirements)” (PDF) and “Part two (Energy Return Factors and Overall Battery Efficiencies)” (PDF). Energy Conversion and Management 46, 2005. ”



Posted by: alain | May 05, 2015 at 01:41 PM
Kris De Decker


@ Alain

“The making of a lithium-ion battery requires between 1.4 and 1.87 MJ/wh” [16, 17, 18]

= 400 kWh energy/kWh of battery storage (based on the figure of 1.4 MJ/Wh)

x 50 GWh of batteries produced annually = 20,000 GWh of energy per year.

Posted by: Kris De Decker | May 05, 2015 at 02:41 PM


“The making of a lithium-ion battery requires between 1.4 and 1.87 MJ/wh” [16, 17, 18] = 400 kWh energy/kWh of battery storage (based on the figure of 1.4 MJ/Wh) x 50 GWh of batteries produced annually = 20,000 GWh of energy per year.”

Thanks, this is correct, found on page 72 of your second link a similar average figure.

The TESLA factory will produce 50 GWh / year.

Knowing each battery can cycle for at least 2000 times during it’s lifetime, thus storing in total around 100 000 GWh over its lifetime, for every year of production (For deep discharge use like in EV cars, if for home energy storage, it will be much higher than 2000 cycles…).

As such the TESLA factory is one gem of a plant, since it will allow on a yearly basis to add 100 000 GWh in overal energy storage capacity to our world, while only consuming 20 000 GWh in energy to get that, from cradle to grave, a return of 500 % on energy investment.

Now the big questions are :

1. is this energy needed to manufacture the batteries from cradle to grave, mostly delivered by ‘clean’ energy, or is it 100% fossil fuel energy ?

2. The batteries will be reloaded with ‘clean’ energy sourced electricity, or from fossil fuel energy sourced electricity ?

3. If for 1. and 2. most of the energy is relatively clean, then we have a big winner here, if for 1. and 2. it is mostly fossil fuel energy, then Tesla is wasting our time building it all.

I have a 4.2kW solar PV panel system on my home roof, and I am supplied by a 100% renewable energy provider utility (ecopower cvba), so for 2. it would be good for my own situation.

I however don’t need home battery storage with my current setup, and do not drive yet an EV, driving range is now way too short compared to fossil fuel cars, and hybrids are still too expensive for me.

By 2020, a hybrid will be my preferred choice, probably, given that battery packs are decreasing 6% in price every year.

Posted by: alain | May 05, 2015 at 07:37 PM


Musk talked about reducing fossil energy use of the US, not the world. I think he meant offshore CO2 release.

Posted by: Peter | May 06, 2015 at 12:03 AM
Sherwood Botsford


A possibly viable storage option:

offers a 15 kWh flywheel for $6K — this is $400/kWh but it can be run to zero power, has an efficiency per cycle of 85%, and should outlast lithium batteries.

And this is an early model. If it follow the typical technology curve the price will drop fairly rapidly as more units are built.

In general the electrical companies need to see themselves in a different light, not as electricity producers, but as distributors.

Posted by: Sherwood Botsford | May 06, 2015 at 10:37 PM


I have no quarrel with your LCAs or even your conclusions.

I do think you’re fundamentally incorrect to assume that off-grid systems will see any uptick at all, even a small one. I also believe the statement I’ve just made is completely different than saying energy storage will or will not continue to grow. The simple fact is that it makes little sense to disconnect oneself from the electricity grid. The grid in North America, and many other parts of the world that are seeing large amounts of solar being installed, is fantastically reliable. Why separate yourself from that? Even if there were some type of fee system imposed on a solar system, I doubt it would be enough to truly consider cutting that cord.

But what you’re not considering, and sort of mentioned in passing in your last paragraph, is the implications of small and large storage becoming part of the grid. People smarter than me have predicted that while the financial incentives and ‘free rides’ of simple grid-connected solar systems might end, smarter utilities will begin to see the value of having “smarter” solar on their system, exactly for the reasons you mentioned (load shaving, frequency regulation, ramp rate adjustments, etc).

In other words, if a solar system is able to automatically respond to what the grid needs with the same level of efficiency as turning a “peaker” power plant off or on, than each kWh it produces should be worth more than the solar system a mile away that is responding to only the level of irradiance.

Examples of this future are starting to show up in island communities, where the grid is often diesel fired and electricity prices are high, though they’re being installed for functionally different reasons — namely that having 3MW of supply disappear when a cloud goes over can wreck havoc on your system.

Still, the ability to shuttle electricity to a large battery bank when supply is saturated, or pull from that bank when needed, is valuable in these situations. In certain places, it’s likely that the only way large solar (in particular) would even be allowed at all is if it can meet some fairly specific ramp rate requirements, and very nearly the only way you can do that is with energy storage.

Will this be the case with smaller systems? It’s hard to say, but it’s I think far more likely than every house on a block being its own disconnected grid.

And regarding performing the LCAs of those type of systems, they’re not really any different. While there may be different electronics, I’m not sure there are MORE electronics in a grid-connected system with batteries compared to an off-grid systems — still charging, inversion, rectification, etc. Once you add batteries into the mix, you’ve already complicated the system. Does a system that talks to the grid really make it that much worse? Probably not measurably — like having your solar factory 50 miles closer to the shipping port.

Thanks for your site — always love the read!

Posted by: Matt | May 07, 2015 at 08:55 PM
Moshe Braner


The claims of 15 years (or more) float lifetime for lithium batteries seems suspicious to me. We’ve had lithium ion batteries in laptops and cellphones and so forth for a while now, and it’s my impression that the batteries never maintain much of their initial capacity by the time they are 10 years old, or even 5. And the number of charge cycles they are subjected to is often far less than the thousands talked about in theory (although in some uses they do undergo daily cycles).

Regarding “Tesla recycles all lithium in batteries” (in a comment above), that is a promise, not a fact. If the batteries indeed last as long as claimed, they won’t have to honor that promise for quite some time, and may be out of business by then.

Finally, do not assume that the batteries, unlike the PV panels, will be made in countries with cleaner grids. If off-grid storage indeed becomes a trend, what’s to stop China from taking over that industry too, like they did to the many previous ones?

Posted by: Moshe Braner | May 14, 2015 at 09:55 PM
Mike Rossetti


Just a few thoughts:
1) I suspect, at least in the short term (‘short’ being 10 years or so) that we are not going to see many net-metering customers go completely off-grid. Rather, we will see them stay on-grid but drop net-metering while employing storage for buffering. Doing so will prevent net-metering customers from being treated as a special class, keeping them in the standard residential customer rate class.
Side note: The utility industry will try to discourage on-grid/buffered residential use through legislation.
Side note: Intelligent buffering reduces the most costly, peak time electricity. And by ‘costly’ I don’t just mean most expensive in terms of dollars but also in the amount of pollutants generated.
2) The assertion in this article that residential solar customers do not pay for their use of the infrastructure is incorrect. This assumption is based on an incorrect claim that solar customers’ peak load is not reduced and, thus, current rate schedules cause cost-shifting to non-solar residential customers. In Utah, the average residential solar customer consumes 5% less energy than does the overall average residential customer, implying that infrastructure usage is not being fairly compensated by those solar customers. However, the average residential solar customer in Utah has reduced their peak load by 7%—meaning they are actually paying for more infrastructure than they are actually using.
Side note: Residential customers taking advantage of high-efficiency trade-in programs also reduce their average consumption but without a demonstrable reduction in peak load.
3) One could ask what the CO2 cost has been of the utility infrastructure (mining ore, smelting, milling, transportation, installation, etc.) but that is all sunk costs so I suppose that doesn’t matter much except for new residential construction.

Posted by: Mike Rossetti | May 15, 2015 at 07:23 PM
Jim Baerg


I would like to make a few points;
You barely mentioned the problem of seasonal variation in solar availability. In a broad band the around equator seasonal variation is small & peak demand is usually in the summer so storage for solar energy is only needed to cover daily variation. Farther from the equator there is little sunlight in winter & demand is high, for solar to be much use at high latitudes the much harder problem of storing months of energy demand would have to be solved.

Where the seasonal variation in solar is small, solar electricity would complement a steady power source like nuclear, with solar covering the daytime peak in demand, especially in the panels are pointed somewhat west of equatorward so their maximum output is in the afternoon when demand peaks. This would minimize the need for storage, which is still expensive.

Re: Bruno (12) Early nuclear reactors had difficulties varying their output due to Xenon 135, but that has to a large extent been solved & the proposed molten salt reactors would not have the problem at all.

This energy storage technology might turn out to be much better than any battery for stationary applications, though it would have too low energy per liter for cars. They claim to have solved the low efficiency of compressed air storage & their method sounds like it should work. Also their compressors & air tanks should last far longer than any chemical battery.

This article ( is worth reading for anyone interested in what can be used to replace fossil fuels.

Posted by: Jim Baerg | May 18, 2015 at 04:43 AM
Russ Finley


I ran the numbers using Tesla’s new battery for solar storage and found that it would cost me well over a million dollars to go off grid using them. But I live in Seattle. The cost will vary considerably by location:

Another engineer ran the numbers for various latitudes and found similar results:

Posted by: Russ Finley | May 24, 2015 at 07:07 PM


well,… the article has a lot of information and misinformation , because a good lead acid battery has about 1500 cycles at 80% dod and about 5000 cycles at 33% dod

and on the otherhand the article starts with a tesla battery picture ….. as known , tesla gives a warranty of only 125000 miles on the 60 kwh sedan battery …… , i would not want this poor battery in my house , to replace the 12 year old lead acid one i have

Posted by: billi | May 26, 2015 at 09:26 AM
Kris De Decker


@ billi (#38)& Moshe Braner (#34):

Li-ion batteries in gadgets don’t last that long because they generally have a DoD of 100%. We usually charge our gadgets only after the battery has died, which limits its durability. For home storage, portability and weight are of less concern so it’s easier to add more batteries and limit the DoD, resulting in improved durability. Electric cars are somewhere in between I guess. This said, the lifespans mentioned in the article are based on a report that assumes intelligent use of batteries. Lifespans will be shorter in real life.

@ Moshe Braner (#34)

Indeed, lithium-ion battery recycling is not a fact because it’s not commercially viable. It will only become so when the easiest lithium supplies are exhausted, and that means that embodied energy will go up anyways. In short, don’t count on recycling to lower the embodied energy of lithium-ion batteries.

@ Jim Baerg (#36):

“You barely mentioned the problem of seasonal variation in solar availability.”

Correct. See note 13. Although it might not seem this way, my article is a very optimistic take on the durability of an off-grid PV solar system. Also check the link in comment #5.

@ Mike Rossetti (#35):

That claim is made by the utilities, not by me.

Posted by: Kris De Decker | May 26, 2015 at 12:43 PM


Bruno – 100% renewable without storage is theoretically possible, it will take a global grid, and then it is always sunny or windy somewhere…

No: When it’s night in Europe and there’s a big anticyclone over the whole area, there won’t be any power anywhere.

Besides, power lines also suffer from NIMBY: The German governement is having a hard time building those to connect wind turbines in the North Sea to users in the West/South of the country.

Just build nuclear plants and be done with it. Getting off oil for transportion is big enough a challenge.

Posted by: YourNameIsRequired | June 08, 2015 at 11:31 PM


This is a very high tech article for Low Tech Magazine.