Originally published October 15, 2013. Updated August 19, 2026.
Capacity factor is the share of its maximum possible output that a power plant produces over a year, and in the US it runs from above 92% for nuclear plants to about 23% for utility-scale solar. Wind sits in between at 34.6%. That spread is why a 100-megawatt solar farm and a 100-megawatt reactor aren’t interchangeable, even though they carry the same rating. It’s also why a grid that leans on wind and solar needs a lot more installed capacity than the fossil and nuclear plants it replaces.
What capacity factor means
The Energy Information Administration defines capacity factor as the ratio of the electricity a plant generates over a period to the maximum it could generate running at full output the whole time, expressed as a percentage.
A 100-megawatt plant running flat out for all 8,760 hours of a year would produce 876,000 megawatt-hours. If it produces 300,000 megawatt-hours instead, its capacity factor is about 34%. A 100-megawatt solar farm at the US average of 23% produces about 201,000 megawatt-hours, roughly two-thirds of that.
Capacity factor is a different thing from efficiency, which describes how much of a fuel’s energy a plant turns into electricity. A wind turbine can be in perfect working order and still post a low capacity factor, because it can only produce as much as the wind gives it. A plant’s nameplate capacity is the most it can produce at any moment, and capacity factor measures how much of that it delivers across a whole year.
US capacity factors by source
The latest US averages from EIA data are below. Most are 2024 figures, and the solar and hydro averages are for 2023.
| Source | Capacity factor |
|---|---|
| Nuclear | Above 92% |
| Geothermal | 65% |
| Natural gas (combined cycle) | 60.5% |
| Coal | 42.6% |
| Wind | 34.6% |
| Hydroelectric | About 34% |
| Solar (utility-scale) | About 23% |
Source: US Energy Information Administration data, compiled by Statista, and the EIA State Energy Data System.
Nuclear plants lead the table by a wide margin, which is part of why they keep coming up in arguments about a low-carbon grid, alongside the questions people still ask about nuclear safety. Coal has dropped hard. In 2009, US coal plants ran at 63.8%, while nuclear ran at 90.3%, according to figures Robert Wilson compiled. Coal’s 42.6% today is roughly two-thirds of that 2009 level.
Hydroelectric numbers move with the weather. The 34% figure is a typical year, and EIA data shows it has ranged from roughly 19% to 40% depending on how wet the year was.
How wind got more productive
US wind turbines produce a lot more of their nameplate rating than they used to. A 2015 report from the National Renewable Energy Laboratory found that the US wind fleet’s average capacity factor rose from below 22% to 32% over the preceding decade. By 2024 it had reached 34.6%.
That rise changes the math for anyone building wind. At 22%, a 100-megawatt wind farm produces about 193,000 megawatt-hours a year. At 34.6%, the same rating produces about 303,000, a gain of more than half from the same nameplate.
Location matters as much as equipment. British government statistics show UK onshore wind ran at a 25.7% load factor in 2024, while offshore wind ran at 38.7%. Per megawatt of rating, Britain’s offshore wind turbines produced about half again as much electricity as its onshore ones.
Country averages differ in the same way. Wilson’s analysis put German wind at about 18% and Danish wind at about 30%. The World Economic Forum drew on the same kind of figures to show that 10 gigawatts of wind in Britain delivers about 2.6 gigawatts on average, against 1.8 gigawatts in Germany.
Solar in the US, the UK and Germany
Solar’s capacity factor depends mostly on how much sun a site gets. US utility-scale solar averages about 23%. The UK, much farther north, posted a solar load factor of 9.8% in 2024, well under half the US figure.
Germany sits close to the UK. In an analysis first published on this site in October 2013, Robert Wilson put German solar below 10%, against about 20% in Arizona. Current numbers are similar. Fraunhofer ISE reports that German solar produced about 87 terawatt-hours in 2025 from 116.8 gigawatts of panels installed by the end of the year. Dividing one by the other gives about 8.5%. The real figure is a little higher, because some of that capacity only came online partway through 2025, but German solar still runs at around a tenth of its rating.
An average like that hides big swings. Wilson pointed out that German solar output peaked at 22 gigawatts in May 2012, about seven times its average output of roughly 3.2 gigawatts. A grid has to absorb those peaks as well as cover the gaps.
Germany has built a huge solar fleet anyway. It’s a good example of how a low capacity factor raises the amount you have to build without stopping a country from building it.
How much capacity it takes to replace a reactor
The same formula shows why capacity factor decides how much a grid has to build. A 1-gigawatt nuclear plant at 92% produces about 8,059 gigawatt-hours a year. Getting that much energy from solar at 23% takes about 4 gigawatts of panels. Getting it from wind at 34.6% takes about 2.7 gigawatts of turbines.
So replacing a retiring reactor with solar means building roughly four times its rating in panels, and replacing it with wind means close to three times. Even then, solar and wind produce on their own schedule, so matching the annual total still leaves the question of what supplies power at night and on calm days.
What changes the tradeoff
Wilson made this argument using 2009 US figures and the grids of Denmark and Germany. His point was that a grid built on low-capacity-factor sources needs far more installed capacity, plus somewhere to send the surplus when the wind and sun peak together. He estimated that past about 70% wind, Denmark would have to export or store around 30% of its wind output, and that Germany would need to curtail or store roughly half its solar output once solar supplied about 40% of its power.
Wind’s capacity factor is up by more than half since the mid-2000s, which shrinks the build-out needed for the same energy. Storage has grown too. US utility-scale battery capacity reached 43.6 gigawatts by the end of 2025, up from about 26 gigawatts a year earlier. Batteries give surplus midday solar somewhere to go besides curtailment, and EIA expects developers to add about 26 more gigawatts in 2027 alone.
