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How Solar PV Can Power A Carbon-Free Energy Revolution, In Four Charts


CREDIT: Shutterstock

Can we build enough carbon-free energy fast enough to avert catastrophic climate change without having to power this energy transition with fossil fuels that would undermine the whole transition? The answer is “yes,” and here’s why.

The “global solar photovoltaic industry is likely now a net energy producer,” concluded a Stanford study released last year. That was followed by a very detailed analysis, Energy Balance of the Global Photovoltaic (PV) Industry, by post-doc Michael Dale and Global Climate & Energy Project director Sally Benson. They examined how much energy is consumed during the entire lifecycle of the production process for every major kind of PV system.

Perhaps their most important conclusion was this:

If current rapid growth rates persist, by 2020 about 10 percent of the world’s electricity could be produced by PV systems … if the energy intensity of PV systems continues to drop at its current learning rate, then by 2020 less than 2 percent of global electricity will be needed to sustain growth of the industry.

As we’ll see below, the energy intensity of solar PV systems has continued to drop in recent years — and is all but certain to continue doing so. That means the solar industry will be generating a vast surplus of carbon free power in the coming years and decades.

Dale and Benson found that the electricity generated by all of the world’s installed solar PV panels in the year 2012 “probably surpassed the amount of energy going into fabricating more modules.” In the figure below, that means 2012 was the “breakeven” year.

Energy inputs and outputs

Energy inputs and outputs for an energy production industry growing asymptotically to some upper limit. Gross output is shown as a bold line; net output is shown with the dashed line.

They projected that “the payback year has a 50 percent likelihood of occurring between 2012 and 2015.” In other words, there’s a good chance the cumulative solar energy generated by every PV system in use as of today equals the cumulative electricity consumed in producing those system to date.

This is “largely due to steadily declining energy inputs required to manufacture and install PV systems.” That is, just as the PV industry has seen a stunning drop in total cost of production — 99 percent in the last quarter century — it has also seen the stunning drop in “energy pay back time” (EPBT) for PV systems. The EPBT is the “time necessary for an energy technology to generate the equivalent amount of primary energy used to produce it.”

This Stanford chart shows that, as of 2010, the energy payback time for PV systems as a whole had dropped to under two years.

Energy Payback

For reasons that are discussed below, the EPBT for PV systems in regions with high amounts of sunlight (high solar insolation), such as the U.S. Southwest, is now under one year.

This year, Dale was lead author on a study that extended the analysis of PV out to 2012 and also examined the wind industry.

Dale et al note that global wind and PV “installed capacities are growing at very high rates (20 percent per year and 60 percent per year, respectively).” Therefore, they “require large, ‘up-front’ energetic investments. Conceptually, as these industries grow, some proportion of their electrical output is ‘re-invested’ to support manufacture and deployment of new generation capacity.”

Here is their chart for the wind industry:

wind payback

Net energy trajectory for the wind industry. The red region represents a net energy deficit and the green region a net energy surplus. Diagonal sloping lines represent the fractional re-investment, i.e. how much of the gross output from the industry is consumed by the growth of the industry.

The red region (a fractional re-investment of greater than 100 percent) “means that the industry consumes more electricity than it produces on an annual basis, i.e. running an energy deficit. The green region represents an energy surplus.”

The wind industry has been in energy surplus for decades. That’s because, relative to PV, wind has had a slower growth rate and a faster energy payback time. Onshore wind has a a fractional re-investment of under 10 percent, which means that over 90 percent of the electrical output of the onshore wind industry is available to society.

The capacity factor is “the average power output [in Watts] of a technology relative to its nameplate capacity [Wavg/Wp].” The wind doesn’t blow all the time, and when it does, it doesn’t always blow as strongly as a turbine is capable of handling. So, the average capacity factor across all wind turbines installed globally is roughly 25 percent. That means each 1 Wp capacity of wind will generate 2.2 kWhe per year. (8760 hours in a year times 0.25 capacity factor = 2200 hours. So 1 Wp capacity generations 2200 W-hours = 2.2 Kwh)

Here is the chart for the solar industry:

Solar chart

Cumulative electricity demand (CEeD) trajectory for the solar PV industry. The red region represents a net electricity deficit and the green region a net electricity surplus. Diagonal sloping lines represent the fractional re-investment. Solar PV has been disaggregated by the major technologies: single crystal silicon (sc-Si), multicrystalline silicon (mc-Si), amorphous silicon (a-Si), ribbon silicon, cadmium telluride (CdTe), and copper indium gallium diselenide (CIGS).

This graph shows that all of the major solar PV technology were in electricity surplus by 2012, except copper indium gallium diselenide (CIGS) and single crystal silicon (sc-Si), which were getting close.

But here is a crucial point about this PV chart and the earlier one. They assume an average capacity factor for solar PV of about 11.5 percent — where 1 Wp of installed capacity will generate 1 kWh per year.

In many parts of the world, such as the U.S. Southwest and Mideast, the actual capacity factor for PV is double that average, over 20 percent. Why do the authors use such a low capacity factor then? They are taking the global average of what is installed. As Dale explains in the news release:

“At the moment, Germany makes up about 40 percent of the installed market, but sunshine in Germany isn’t that great. So from a system perspective, it may be better to deploy PV systems where there is more sunshine.”

The energy payback time of solar systems can be reduced in two ways. First, the world can continue improving the technology and cutting costs as it has for decades. Second, the world can install a larger fraction of PV panels “in locations with high quality solar resources, like the desert Southwest in the United States and the Middle East.

In fact, as Climate Progress reported last week, the price of utility-scale solar power is 59 percent lower than analysts projected it would be just four years ago, according to a report from two U.S. national labs. Indeed, the price of a roof-top solar system dropped 12 to 15 percent between 2012 and 2013 alone.

This means the astonishing growth in solar capacity in the United States since 2010 is very likely occurring with systems that have an energy payback of under one year.

Bottom Line: We can certainly build enough carbon-free power systems fast enough to avert catastrophic warming without having to power that energy transition with fossil fuels that would undermine the transition.

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Keith Pickering's picture
Keith Pickering on October 31, 2014

This is an interesting take on EROI, but I would caution that simply tipping over from the red area to the green area won’t be enough; that’s essentially an industry-wide EROI of 1, which isn’t close to the economic limit. To get a rough idea of the industry-wide EROI from Dale’s graph, take the reciprocal of the fractional re-investment (the diagonal lines). Thus we see wind with a fractional re-investment of about 6% in 2012, which corresponds to an industry-wide EROI of 17, roughly the same as other studies.

Weissbach et al. put the economic limit of EROI needed to avoid economic contraction at 7, which implies a re-investment level of 14%. PV technologies are moving in the right direction, but it’s clear we still have quite a ways to go.

Hops Gegangen's picture
Hops Gegangen on October 31, 2014


“…it may be better to deploy PV systems where there is more sunshine.”


Nathan Wilson's picture
Nathan Wilson on October 31, 2014

It doesn’t make sense to say an energy source can power a revolution if it is limited to low penetration use.  A 10% contribution of sustainable energy (e.g. an aggressive solar deployment) is not good enough.  

We need 90-100% sustainable energy.  Solar can contribute to that during the day,  in warm, sunny climates.  But adding batteries for night time use is problematic economically and for EROI.  And in colder climates (with wintertime energy demand peaks), solar is just in the way of other clean energy sources.

Robert Bernal's picture
Robert Bernal on October 31, 2014

The last statement is threatened by lack of subsidy and lack of storage potential. I’m sure there is an equation which explains how subsidy must be less and less per unit as growth exponentiates. This can only be overcome by an even larger reduction of overall installation prices and likewise with the other 78% – storage, thus requiring substantually less costs (again) in order to come close to what other reliable sources can do – without hydrocarbons.

Of course, we also have to make sure that as the Eroei goes up, that the Esoi (of storage, the main ingredient) to also be high. Lead acid is somewhere below 5 (some say only 2 but I don’t believe it as almost all lead acids are recycled). Pumped hydro and powerlines are somewhere around 100!

Bob Meinetz's picture
Bob Meinetz on November 3, 2014

Joe, I don’t believe I’ve ever come across a more vivid example of the denial at the heart of renewables advocacy than this post. The yawning logical gulf between

Bottom Line: We can certainly build enough carbon-free power systems fast enough to avert catastrophic warming without having to power that energy transition with fossil fuels that would undermine the transition.

and everything that precedes it is a textbook example of how renewables’ heady allure can make otherwise-intelligent individuals take leave of their senses (below, Nathan explains why this “bottom line” is not a justified conclusion in any rational argument).

I’m relatively certain the carbon resulting from printing pretty graphs at Mark Jacobson’s Stanford Antinuclear Propaganda Factory outweighs the benefits of all the solar in the country – if, on a net basis, there indeed are any.

Robert Bernal's picture
Robert Bernal on November 3, 2014

I like the advances in solar and wind technology, however, we need to reduce fossil fuels usage by like 90% – globally. Solar is great for rooftops and such to power a substantial portion of the residential sector during its generation within it very limited Capacity Factor. However, the inverse of its CF must be stored (ever sized on off grid system?). Now, fatham what it takes to power a world of over 10 billion people at high standards with renewable energy…

The world consumes over 500 exajoules of primary energy. If generated from 45% wind, 45% solar and 10% hydroelectric, that amount would be down to less than 200 exajoules assuming electric cars and trucks, too. The world needs at least 4 multiples of the power we consume today to become properly developed. We need even more to properly advance and clean up the excess CO2 mess. This puts us at about 1,000 exajoules from non fossil backed RE sources. This equates to how much land? At least an additional 1% of ALL land for solar and millions of miles of ocean crossing power lines for the wind component.

Now, add in the storage. Remember, that since RE has only about 25% CF, the inverse of that has to be stored. I believe this converts to an overbuild of about 50 TW, most of which has to be stored for at least a day. Pumped Hydro Storage (and lots of power lines) is the cheapest bulk storage. I conclude that such a developed world based on RE alone would need about 100,000 1 km x 2 km x 30 meter lakes AND a lower storage reservoir. This is a “no no” in the eyes of environmentalists!

Obviously, there is a better solution for an advanced planetary civilization. that could be OTEC (since it would be 24/7) and definitely, advanced nuclear (as already proven)!

Bob Meinetz's picture
Bob Meinetz on November 3, 2014

Job001, first of all you’re making an implicit assumption that utility guys are making a mistake. In most cases, I think they see renewable energy as a technical and financial loser but one with green appeal in the media. They’re trying not to alienate people who aren’t familiar with the technical aspects of power generation, while still remaining profitable.

I’m not sure what you mean by “the competition is negligible”. The definition of a monopoly is that there is no competition, and it’s under that premise that a majority of utilities in the U.S. operate. To qualify, they tolerate strict regulation of pricing and are overseen by PUCs, which can fine them for breaches of their duty to provide the public with reliable power. In extreme cases they can lose their monopoly.

But the idea that there are “required changes” is a misconception. If the changes you want are going to result in reliability or profitability problems for utilities they will never, ever happen. Unfortunately, I think most of the changes for which solar advocacy is pushing fall into that category, and solar advocates need the grid just as much as everyone else.

Robert Bernal's picture
Robert Bernal on November 3, 2014

Edited: I’m NOT pretending to be an expert. I’m displaying basic math. We ALL should take the basic energy math serious. Nobody has to be utility qualified or a mathematical genius to know “solar and wind alone will require vast amounts of land” past normally acceptable limits.

Bob Meinetz's picture
Bob Meinetz on November 3, 2014

Job001, I have never met a utility guy who thinks like the stereotype you describe, and my acquaintances in that field range from plant maintenance engineers to the former president of Southern California Edison. Innovation happens on a daily basis, and the ones I know are among the brightest and most dedicated people you’d want to meet. Most research and development happens at contractors – just like it does for solar and wind – but utility engineers are constantly working on ways to improve efficiency, and take a lot of pride in making generation as “green” as they possibly can.

You seem frustrated that what you consider the greenest alternatives are not being implemented fast enough, but there are very good reasons for why certain modes of generation are favored over others. They might be reasons of economy, reliability, or environmental impact – usually a combination of all three – but believe me, utility engineers who lack imagination, creativity, or a get-it-done ethos don’t last very long. They work with all of the technologies you describe above, and any new developments which are competitive get a fair shot – it would be completely counterproductive (not to mention unprofitable) to do it any other way.

You should take a tour of a modern utility generating facility, you would be surprised. The bar is higher than you think it is.

Peter Lang's picture
Peter Lang on November 6, 2014

It seems to me renewables with energy storage is not sustainable.  It seems the energy return on energy invested (ERoEI) in renewable energy and energy storage is not sufficient to supply the energy needs of modern society. . 


 The EROEI needs to be at least 14 to support modern society.  So, only fossil fuels, hydro and nuclear can do it.


Below are some ERoEI figures for various electricity generation technologies.  These include buffering – i.e. energy storage so the unreliable, non dispatchable renewables are properly comparable with the dispatchable technologies.


Solar PV = 1.6


Biomass = 3.5

Wind = 3.9

Solar CSP (desert) = 9

Gas (CCGT) = 28

Coal = 30

Hydro = 35

Nuclear = 75


 ERoEI has been extensively debated in the litterature and is very sensitive to assumptions inputs and methodology.  However, this seems to be an authoritative and widely cited source.


 It seems to me, if this is correct, it means renewables are not sustainable.


Bob Meinetz's picture
Bob Meinetz on November 7, 2014

Richard, the heat created by energy production, on a global scale, is not a problem. The earth radiates all but a tiny percentage of radiation received by the sun – far and away the biggest source of terrestrial heat energy – back out to space.

The trapping of solar energy by GHGs is a serious problem.

Robert Bernal's picture
Robert Bernal on November 7, 2014

It’s the other way around. Our total energy consumption is orders of magnitude less than that of Earth’s solar insolation. However, our GHG’s are on course to double the natural amount necessary for the planet at this geological time frame. Remember, as seen from physics POV, all energy is wasted (as heat). I do believe in efficiency, such as a billion nuclear charged electric cars to replace the gas burners, to replace coal fired plants and to not have to rely on fossil fuels to back up the 70% or so when wind and solar are not generating.

Robert Bernal's picture
Robert Bernal on November 7, 2014

We are talking about the inability of solar and wind to back themselves 95% in a non fossil fueled world. They are diffuse, intermittent and thus need to be seen as what they are really for – partial amounts of power and for learning with.

Peter Lang's picture
Peter Lang on November 9, 2014

Job001 Gibson

Your comment doesn’t seem very persuasive compared with the mass of discussion I’ve seen by apparently seemingly appropriately qualified people who have discussed the report.  Might I suggest you post your comment on the thread at BraveNewClimate and debate your points with others.

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