Originally published September 27, 2017. Updated September 16, 2026.
The International Energy Agency’s 2013 World Energy Outlook expected the world to add about 28 gigawatts of solar power a year by 2018, but the actual pace that year ran three to four times higher, closer to 100 to 110 gigawatts a year. That gap is why Wikipedia and outside researchers still point to the IEA when the subject is forecasters missing solar’s growth. The world added 452 gigawatts of new solar in 2024 alone. But the agency’s newest outlook, published in 2025, doesn’t repeat that pattern. For the first time in years, it lowered its five-year forecast, citing policy shifts in the US and China.
How far short the early forecasts fell
Adam Whitmore, an energy policy analyst who has tracked IEA forecasts since 2013, laid out the scale of the early miss in a 2019 post. The 2013 outlook expected around 28 gigawatts of solar installed per year by 2018, while actual installations that year were already running at 100 to 110 gigawatts annually. A 2025 academic review of IEA World Energy Outlooks published between 1993 and 2022 found that renewable energy growth was underestimated across nearly all of them, in an analysis by LUT University researchers reported by pv magazine.
| IEA forecast | What it projected | What happened |
|---|---|---|
| 2013 World Energy Outlook, for 2018 | About 28 GW of solar added per year | 100 to 110 GW added per year |
| 2020 World Energy Outlook, five-year total for 2022 to 2026 | 1,092 GW of new renewable capacity | Revised up to 1,920 GW two editions later, while solar alone added 345 GW in 2023 and 452 GW in 2024 |
| 2025 World Energy Outlook (Renewables 2025), for 2025 to 2030 | About 3,680 GW of new solar capacity | Revised down slightly from the prior year’s forecast |
Source: Adam Whitmore (2019). Carbon Brief. IRENA. IEA, Renewables 2025.
How much solar the world built in 2023 and 2024
By the numbers that matter, solar’s growth kept accelerating while the forecasts were catching up. Global solar capacity additions reached 345.5 gigawatts in 2023, based on IRENA data reported by pv magazine, then climbed to roughly 452 gigawatts in 2024, according to IRENA’s own tally. Total renewable capacity added in 2024, across solar and every other renewable technology combined, came to 585 gigawatts. Cumulative installed solar capacity crossed 1 terawatt in 2022 and then doubled to 2 terawatts two years later, according to Ember’s Global Electricity Review.
Solar generation grew fast too. It’s a different measure than installed capacity, since how much of that capacity turns into electricity depends on capacity factor. Solar output grew by 474 terawatt-hours in 2024, up 29% year over year, which Ember calls the largest source of new electricity generation worldwide for the third year running. The first half of 2025 kept that pace, with solar generation up 306 terawatt-hours, or 31%, and solar’s share of global electricity rising from 6.9% to 8.8% in a single year.
What the IEA expects for 2025 through 2030
The IEA’s latest outlook, Renewables 2025, projects 4,600 gigawatts of new renewable capacity worldwide between 2025 and 2030, with solar PV accounting for about 80% of that, or roughly 3,680 gigawatts. That’s still a forecast for rapid growth. It’s also lower than the agency’s prior outlook. The IEA revised the number down for the first time in years, pointing to policy shifts in the United States and China as the reason. Al Jazeera covered the same revision under a sharper headline, describing it as a sharp fall in the global outlook for renewable power.
Why the models kept missing solar’s growth
Part of the miss came down to price. Adam Whitmore and Terje Osmundsen, then a senior executive at the Norwegian solar developer Scatec Solar, argued in 2017 that the IEA’s solar cost assumptions ran about double the market prices solar was selling at, which made solar deployment look far more dependent on subsidy than it was. In his original 2013 analysis, Whitmore made a related argument. The agency’s models leaned on linear extrapolation of national plans at a time when solar costs and installations were both moving on an exponential curve.
That gap between what’s affordable and what the models expect still shows up in long-range scenarios. The IEA’s own 2022 net-zero pathway projects 15.5 terawatts of solar by 2050, while a LUT University analysis reported by pv magazine put what’s technically and economically feasible at 63.4 terawatts, about four times higher.
Has the agency fixed the problem, or changed direction
The IEA did correct course for a while. Between 2020 and 2022, it raised its own five-year renewable capacity forecast across three consecutive editions, from 1,092 gigawatts to 1,496 gigawatts to 1,920 gigawatts, a 76% increase in two years, according to Carbon Brief’s analysis of the agency’s own reports. Solar installers kept beating even those newly raised numbers in the years that followed, adding 345 gigawatts in 2023 and 452 gigawatts in 2024.
The 2025 outlook breaks that pattern. The IEA’s five-year forecast moved down for once, and it points to policy retrenchment in the US and China as the cause. That’s a different kind of miss than the one Whitmore and the LUT researchers described. The next few years of installation data will show whether the lower number holds up or whether the agency has once again misjudged how fast solar keeps building.
What the forecast gap has meant for policy and investment
The 2017 argument from Whitmore and Osmundsen went beyond accuracy for its own sake. Their case, as Wikipedia’s summary of the piece describes it, was that cost assumptions running about double real market prices produced projections that made solar growth look dependent on heavy subsidy, a framing they said risked discouraging investment and slowing deployment. Actual installations kept accelerating through the years that followed regardless, as the capacity and generation numbers above show.
Grid operators now face a question the original pieces didn’t have to answer in the same way. They have to manage power generated when demand is low and the sun is still out, largely through battery storage, and that balancing act is now as much a part of the growth story as the installation totals themselves.
What would change this picture next
The clearest test is whether the IEA’s 2025 downgrade holds up against actual installations. Solar generation was still up 31% year over year in the first half of 2025. If full-year 2025 and 2026 numbers come in below the lowered forecast, that would mark the first stretch since Whitmore’s original analysis where the agency’s solar number ran too high. If they come in above it again, the record stays what it’s been since 2013, with solar beating the IEA’s forecasts.
