Capacity Factor Explained

Capacity factor is the share of a power plant’s theoretical maximum output that it actually produces over a period of time. Divide the energy a plant generated in a year by the energy it would have generated running at full rating for every hour of that year, and the answer is its capacity factor. It is the single number that explains why a gigawatt of one technology and a gigawatt of another deliver completely different amounts of electricity.

The arithmetic

A plant’s rated capacity is measured in megawatts, a rate. The electricity it produces is measured in megawatt hours, a quantity. There are 8,760 hours in a normal year, so a 100 megawatt plant could in principle produce 876,000 megawatt hours.

If it actually produced 300,000, its capacity factor is 300,000 divided by 876,000, or about 34%.

That is the whole calculation. The interest lies entirely in why the number comes out where it does, because two very different causes are mixed together in it.

The first cause is availability: whether the plant could have run. A reactor in a refuelling outage, a turbine under repair or a wind farm on a still day could not.

The second is dispatch: whether it was worth running. A gas plant that sat idle for eight months because the market price was below its fuel cost was perfectly capable of producing and chose not to.

A capacity factor mixes both, so it describes what happened rather than what was possible.

Typical ranges by technology

The numbers below are broad general ranges. Real values vary a great deal between individual plants, countries and years, so treat them as orders of magnitude rather than targets.

Nuclear is usually the highest of the large technologies, commonly 70% to 90% or above where reactors are run steadily and outages are well managed. It drops where reactors follow demand, where outages run long, or where a fleet is unusually old or unusually new.

Geothermal and bioenergy can also be high, often 50% to 90%, since both burn or tap a resource that is available continuously.

Coal and gas span almost the entire range, and their capacity factors are set by economics rather than by the machines. A modern combined cycle gas plant in a market where gas is cheap may run most of the year. The same design in a market full of renewables may run a fraction of it. Open cycle peaking plants are built specifically to run rarely, and capacity factors in the low single digits are normal and intended.

Hydro depends on the type and the weather. Reservoir plants sized for peak output commonly land somewhere between 25% and 50%, run of river plants higher but at the mercy of the season, and a drought year can move a whole country’s number sharply.

Offshore wind is typically 40% to 55% in good locations, because wind at sea is stronger and steadier and modern turbines are very large.

Onshore wind is typically 25% to 40%, with newer machines at the upper end and older or poorly sited ones below.

Solar is the lowest, roughly 10% to 25%, for the unavoidable reason that half of every day is dark. Northern Europe sits near the bottom of that range and sunny low latitude deserts near the top.

Why capacity is not generation

This is the practical point behind the whole idea. Headlines announce capacity, in gigawatts, because that is what gets built and financed. What people actually care about is generation, in terawatt hours, because that is what gets used.

The gap between them is large. A gigawatt of solar at a 15% capacity factor and a gigawatt of nuclear at 85% differ by nearly six times in annual output. A country that adds more solar capacity than nuclear capacity in a given year may still add less solar electricity.

The same mismatch appears when comparing countries. A large installed base of a low capacity factor technology can look impressive on a capacity chart and modest on a generation chart. Ember’s statistics show solar producing about 9% of world electricity in 2025, a figure well below solar’s share of the world’s installed capacity, and the difference is capacity factor and nothing else.

Our fuel pages and country pages show generation, capacity and shares side by side for exactly this reason. Reading only one of them gives a misleading picture.

Four common traps

The denominator is a choice. Solar farms often install more panel capacity than their inverters can pass through, so the same site has a higher capacity factor measured against the inverter rating than against the panel rating. Always check which rating is being used before comparing two numbers.

The period matters. A capacity factor for a single sunny month, or a single windy quarter, is not comparable to an annual one. A solar farm’s summer figure can be double its winter figure in a temperate country.

Curtailment pushes it down. When a grid cannot absorb what a wind or solar farm could produce, output is reduced and the capacity factor falls even though nothing is wrong with the equipment. This becomes more common as renewable shares rise.

Capacity factor is not capacity credit. Capacity credit is how much firm, dependable capacity a plant contributes at the moments the system is most stressed, and it is a different and usually smaller number. A solar farm with a respectable capacity factor contributes nothing at all on a winter evening peak after sunset.

What a good capacity factor is

There is no single answer, because the question is really about economics. A plant is worth building if the electricity it sells is worth more than it cost to build and run, and that comparison depends on when the output arrives, not only how much of it there is.

A peaking plant running 3% of the year can be perfectly viable if those hours are the most valuable ones. A wind farm running 45% of the year can struggle if most of its output lands in hours when the price is near zero.

Capacity factor is a useful description. It is not, on its own, a verdict. For how the figures on this site are compiled and what periods they cover, see our methodology page.