Bill McKibben has a new book on solar energy coming out in a couple of weeks, and the New Yorker has published a nice article, drawn from portions of the book, that includes this remarkable passage:
But here’s the current prediction from the I.E.A.: by 2026, solar will generate more electricity than all the world’s nuclear plants combined. By 2029, it will generate more than all the hydro dams. By 2031, it will have outstripped gas and, by 2032, coal. According to the I.E.A., solar is likely to become the world’s primary source of all energy, not just electricity, by 2035.
Sounds impressive, right? But to see just how impressive it is, let’s add some context. (And let’s hope I’m not merely duplicating material that’s in the rest of McKibben’s book.)
IEA projections
By “current prediction from the I.E.A.”, I think McKibben means the Stated Policies Scenario in the International Energy Agency’s most recent World Energy Outlook, published in October 2024. There’s a chart on their web site that shows this scenario’s solar generation through 2035, alongside the other major electricity sources. Here’s a plot of the same numbers, adding in a little more history and the three minor sources:
Sure enough, just as McKibben says, this projection shows solar soaring past nuclear, hydro, gas, and coal, in rapid succession, over the coming decade. A couple of the crossing dates are slightly different from the dates McKibben gives, but the discrepancies are too small to quibble over. The IEA Stated Policies Scenario unambiguously puts solar in first place among electricity sources less than a decade from now. What’s more, it puts wind in second place.
But electricity generation isn’t merely a race to see who comes in first and second. We also want the total amount of electricity to grow, so poorer regions can improve their living standards and everyone can replace polluting combustion fuels with electricity. At the same time, we want to reduce the world’s use of the more damaging sources of electricity. To assess both of these forms of progress, it’s useful to plot the same numbers on a stacked area chart:
Here we can more readily see that in this scenario the world’s total electricity use continues its steady climb. And we can see that as a fraction of total electricity, the share provided by fossil fuels declines from well over half today to about a third in 2035. That’s real progress! Yet either chart also shows that the total amount of electricity from fossil fuels declines only modestly by 2035, with coal dropping by about a third while gas hardly changes. Deep decarbonization of global electricity is a goal for subsequent decades.
Meanwhile, although the IEA projection has solar well out in first place by 2035, solar then accounts for only a quarter of all electricity generation. That’s a spectacular increase from just 8% in 2025, but not enough by itself to provide all the projected growth in total electricity generation—let alone to reduce fossil fuel use.
A reality check
A separate question is whether the IEA’s Stated Policies Scenario is a realistic projection of what will actually happen. I’m not qualified to assess it in any detail, but here’s one reality check. BloombergNEF publishes detailed projections of new solar capacity additions, based on economic and other factors in each regional market. And unlike the IEA, they’re being paid by their commercial clients to get these projections right. Jenny Chase, their solar guru, has generously shared their projection from the third quarter of 2024:
Here we can see that solar’s era of exponential growth has ended. BNEF projects that annual installations will continue to increase, but only linearly, and more slowly than they did in 2023 and 2024.
To translate these new capacity installations into amounts of electrical energy generated, we need to apply a capacity factor: the percentage of peak power that the installed panels generate, on average, over an entire year. For existing solar installations, as of 2024, the worldwide average capacity factor is 12.3%. (To get this number I divided the 2024 world solar generation, 2129 TWh, by the number of hours in a year to get an average solar-generated power of 242 GW. Then I divided by the total installed solar DC capacity as of the middle of 2024, 1960 GW, which I obtained by adding up the annual values in the chart above and an earlier version of it that goes back to 2007. Note that DC capacities are always larger than AC capacities, so DC capacity factors are smaller than AC capacity factors. I'm using DC because BNEF does.)
If we apply the 12.3% capacity factor to BNEF’s predicted new capacity over the next decade, we get an estimated total solar generation that’s slightly above the IEA Stated Policies Scenario:
This near-agreement makes me much more confident that IEA’s solar projection is close to what we can actually expect.
IEA vs. EIA
On the other hand, there’s another highly reputable agency that has projected much lower numbers. The US Energy Information Administration published its most recent International Energy Outlook back in October 2023, with projections for a “reference case” and six “side cases”: high and low economic growth, high and low oil prices, and high and low zero-carbon technology prices. Here I compare1 the IEA and EIA projections for world solar generation, with EIA’s reference case in red and shading to indicate the full range of the six side cases:
At least one of these two projections is going to turn out to be badly wrong.
Just as troubling as the huge gap between the two projections is the ridiculously narrow range among the seven EIA cases. I genuinely admire anyone who’s willing to stick their neck out with a quantitative projection of what the future may hold. I have even more admiration for a projection that comes with an associated range of uncertainty. But I see no point in suggesting that the uncertainty range is far smaller than everyone knows it actually is.
But with or without its side cases, I don’t believe the EIA solar projection—and I don’t think this is merely because I dislike it. To highlight what’s wrong with it, I’ve calculated the new solar capacity that it implies would be added each year, again assuming a 12.3% capacity factor:
Here the black bars are from the BNEF data shown above, while the red bars are the annual capacity additions implied by the EIA reference case generation data, assuming that the DC capacity factor remains at 12.3%.2 The tops of the gray bars indicate the actual capacity additions in the two years that have ended since EIA’s International Energy Outlook was published.
The enormous gap between the two gray bars and the red bars below them shows that EIA failed badly at assessing the state of the solar industry as of October 2023. That’s one reason to distrust EIA’s solar projection. Then, looking ahead to the coming decade, EIA projected a slowdown in the rate of new installations, even relative to the modest 2025 peak in its own projection. Relative to the actual 2025 installation rate, that predicted slowdown would be an abrupt crash, from around 600 MW to just 200 MW installed each year.
I'm open to the possibility that the current solar boom won’t last forever. Perhaps we should expect a slowdown as solar penetration levels increase and the intermittency of sunlight makes further installations less profitable. But I can’t think of why a slowdown would occur so suddenly, at a time when solar still provides less than 10% of global electricity. (In the EIA reference case, solar would reach only 13% of global electricity generation by 2035.)
So I’ll stick out my own neck now and predict that world solar generation in 2035 will be closer to IEA’s Stated Policies Scenario than to EIA’s reference case. Even if I’m only barely right, that would put solar in first place among electricity sources by 2035—if IEA’s projections for the other sources turn out to be correct. But what about those other sources?
Other electricity sources
I think almost everyone would agree that nuclear and hydro will grow only slowly over the next few years, so solar should easily pass both of them sooner rather than later, as McKibben says. As for the fossil fuels, I don’t know enough to form a strong opinion. Let’s just say it wouldn’t surprise me to see coal plateauing (as EIA projects) rather than falling (as IEA projects), and/or gas rising gradually rather than plateauing (as both IEA and EIA project). Then, if the growth of solar is on the low side, it might not pass coal or gas until after 2035. For coal, it could take as much as another decade or so. But I think it’s more likely that solar generation will pass both gas and coal by around 2035.
Incidentally, I think the IEA projection for wind energy is almost certainly too high. I’ll be surprised if wind generation exceeds either gas or coal by 2035. The wind industry is currently facing several challenges, one of which is competition from the booming solar industry:
Total energy
So far I’ve been talking only about electricity—not total energy. But the final claim in the McKibben quote above is that “According to the I.E.A., solar is likely to become the world’s primary source of all energy, not just electricity, by 2035.”
I’m unsure of how to interpret this claim, and I can’t find anything similar to it in the IEA World Energy Outlook. But let’s consider some ways in which it might or might not be accurate. I’ll start by comparing solar to natural gas.
Tables A.12 and A.13 of the IEA World Energy Outlook project that, in the Stated Policies Scenario, world production and consumption of natural gas in 2035 will be 4,422 billion cubic meters. In energy units, according to the conversion factor on page 356, that’s about 44,000 TWh, which is more than four times as much as the projected solar electricity production (10,750 TWh) shown in the first chart above. If solar provides less than a quarter as much energy as natural gas, it sure wouldn’t seem accurate to call it the “primary source of all energy”.
Comparing fossil energy to renewable electricity in the way I just did is unfortunately all too common. It’s also grossly unfair, because I’ve credited solar energy only for the electricity it produces, while crediting gas for the heat we get when it burns. If you use that heat to generate electricity at a power plant, you typically get less than half a TWh of electricity out for each TWh of heat you put in—thanks to the inevitable thermal losses from heat engines.
The traditional remedy for the unfairness of this “direct method” of comparison is to divide the solar electricity generation by the average efficiency of fossil electricity generation, to obtain solar’s “primary energy by the substitution method”: the amount of fossil fuel that you would need to burn in order to produce the same amount of electricity. Nowadays the average efficiency of fossil power plants is about 40%, or 0.40. So solar’s (projected) primary energy by the substitution method in 2035 would be 10,750 TWh / 0.4, that is, 10,750 TWh times 2.5, or about 26,900 TWh. And that’s still considerably less than the 44,000 TWh projected to be coming from gas. (Alternatively, we could multiply 44,000 TWh by 0.4 to get the amount of electrical energy the gas could generate, 17,600 TWh, and compare this to the 10,750 coming from solar. Again, gas comes out well ahead.)
I suppose you could try to argue that the efficiency factor of 0.40 is too high, because some gas is burned for purposes (like heating buildings) that could be served by electrical appliances (like heat pumps) that provide efficiency improvement factors of more than 2.5 (in some cases). But I don’t think it would be reasonable to argue that the average efficiency factor for all natural gas use should be as low as one fourth, especially because combined-cycle power plants have efficiencies somewhat above 40%.
Instead of comparing solar to gas, we could compare it to oil. Again, in the IEA WEO Stated Policies Scenario, oil tends to come out ahead—though the calculation is complicated by the ambiguities of what efficiency factor to use, and whether “oil” should include natural gas liquids, biofuel admixtures, and/or non-energy uses of liquid hydrocarbons such as production of plastics. If you split oil into sub-categories like gasoline, diesel fuel, jet fuel, and heating oil, then each of them separately is projected to provide less energy than solar by 2035, once we apply an efficiency correction as described above. But I’d be surprised if McKibben secretly had that kind of splitting in mind.
More likely, I think, is that McKibben simply made a mistake when he tried to compare solar electricity to total energy coming from oil or gas. Or perhaps he got the talking point from someone else who made a mistake. (Or perhaps I’ve made a mistake in the calculations I’ve just described—in which case I hope someone will tell me!)
There is one more sense in which solar will be the world’s primary energy source in 2035: the same sense in which it has been the world’s primary energy source for billions of years. Each year, the sun delivers just over one billion TWh of radiant energy to earth’s surface and atmosphere,3 warming our planet to a temperature that can sustain life. We tend to take this huge energy influx for granted; neither IEA nor EIA nor anyone else includes it in their official global energy statistics. Even when we design buildings with passive solar heating features, nobody tries to add up the absorbed solar energy on a society-wide scale—probably because there’s no consistent, feasible, or useful way to do the accounting.
I’m quite sure, in any case, that these long-standing uses of solar energy were not what McKibben had in mind when he said solar would become our primary source of energy by 2035. Still, we should remember that all those solar panels we expect to have by 2035 will be intercepting less than 1/10,000 of all the sunlight that hits earth’s surface.4 There’ll be plenty of room to expand them further until solar electricity really does provide more total energy than fossil fuels.
Update, 19 August 2025
Now that McKibben’s book has been published, I’ve corrected the first sentence of this article to indicate that McKibben’s New Yorker article is not actually an excerpt from the book (as I originally said) but rather is “drawn from” it.
The passage in the book that corresponds to the passage I’ve analyzed here is rather different. It reads:
By 2026, according to the International Energy Agency, solar will generate more electricity than all the world’s nuclear plants combined. By 2028 it will generate more than all the hydro dams. By 2030 it will have outstripped gas, and by 2032 coal. If we manage to stay on the IEA’s net-zero-by-2050 curve, then solar energy will become the earth’s largest source of all energy, not just electricity, by the 2040s.
Notice that the exact dates when solar is projected to surpass hydro and gas are different in this version. But again, these details are too minor to quibble over.
More importantly, the final claim in this version, comparing solar to other sources of total energy (not just electricity), is completely different. Here (in the book version), we’re talking not about the IEA Stated Policies Scenario, but rather about the (far-fetched, in my opinion) Net Zero Emissions by 2050 Scenario. Also, the date when solar is projected to achieve first place is not 2035 but “the 2040s”. This is a far weaker, and less notable, claim than the one in the New Yorker article.
1It’s a little unfair to compare an IEA projection from 2024 to an EIA projection from 2023, but these are the most recent versions from each agency. There is, though, a 2023 version of the IEA World Energy Outlook, in which the Stated Policies Scenario projects solar electricity generation in 2035 at 8,748 TWh (see Table A.3a, page 267). This is substantially lower than the corresponding 2024 projection of 10,747, though still far above the EIA reference case projection of just 4,531 TWh.
2The EIA International Energy Outlook includes projections of generation capacity, including solar. I haven't used those projections here because they seem to be for AC rather than DC capacity, making comparisons to BNEF’s numbers difficult, and because they seem to assume a much higher capacity factor for future installations than for past installations. Let me also note here that there is some minor ambiguity in assigning capacity additions to particular years, because new capacity added mid-year contributes only partially to generation for that year. So the height of any particular red bar on this chart could be adjusted up or down a little, if an adjacent bar is adjusted to compensate.
3Not counting another 450 million TWh that gets reflected back out to space, mostly by clouds.
4At 20% efficiency, the panels intercept five times as much radiant energy as the electrical energy they generate, or about 55,000 TWh if IEA’s projection for 2035 is correct. The solar radiation striking earth’s surface is about 750 million TWh per year (less than a billion TWh because some of the billion gets absorbed by the atmosphere). Dividing 750 million by 55,000 gives about 14,000, which I’ve rounded to 10,000 to be on the safe side.
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