Capacity Factor Explained: Why 100 MW of Solar Is Not 100 MW

Two announcements, both plausible: a country installs 100 MW of solar, and another installs 100 MW of gas. The headline numbers match. The electricity delivered does not, and it is not close.

Capacity factor is the number that makes the comparison honest.

The definition

Capacity factor is actual energy produced over a period, divided by the energy that would have been produced running at full rated output for the whole period.

A 100 MW plant running flat out for a year would produce 876,000 megawatt-hours. If it actually produced 219,000, its capacity factor is 25 percent.

That is all it is: a utilisation measure, expressed as a percentage.

Typical values

Approximate ranges, which vary considerably by site and market:

• Solar photovoltaic: roughly 15 to 25 percent. Sunny regions with tracking reach the upper end; temperate, cloudier locations the lower.
• Onshore wind: roughly 25 to 45 percent, heavily dependent on site quality.
• Offshore wind: roughly 40 to 55 percent, reflecting stronger and steadier resource, as we discuss in onshore versus offshore wind.
• Nuclear: typically 80 to 95 percent, designed to run continuously.
• Combined-cycle gas: varies enormously — technically capable of high utilisation, but frequently run far less depending on market conditions.
• Hydropower: varies with water availability and whether the plant has storage.

Solar's ceiling is straightforward: it is dark at night. Even a perfect desert site with no clouds, no dust and perfect equipment cannot exceed what daylight allows.

What it is used for

Converting capacity into energy. Capacity in megawatts tells you nothing about delivered energy without a capacity factor. This is the single most common error in energy reporting, and it makes renewable capacity announcements look larger than their contribution.

Comparing sites. Two wind sites with identical turbines can differ substantially in capacity factor, which translates directly into revenue. This is why wind resource measurement campaigns are worth their cost.

Project finance. Lenders model revenue from expected output. Capacity factor assumptions drive the whole financial model, and small changes move debt sizing materially.

System planning. Planners need energy, not capacity. Replacing a gas plant requires enough renewable capacity to match its energy output — which, given the capacity factor gap, means several times the nameplate figure, plus storage or firm capacity for timing.

Why high is not automatically better

This is the subtlety that matters commercially.

Capacity factor measures how much, not when. A plant with a lower capacity factor delivering electricity during peak demand can be worth more than one with a higher factor delivering into a surplus.

Consider a solar-heavy grid. Additional midday solar arrives when the system already has plenty, so its marginal value is low. Additional evening generation arrives when supply is scarce, so its value is high — even from a plant that runs only a few hours.

This is why single-axis trackers are valued beyond their raw energy gain — they push output toward more valuable hours, as we explain in solar trackers explained — and why storage is increasingly procured alongside solar across the Gulf.

When capacity factor is misleading

Curtailment. A wind farm instructed to reduce output because the network cannot carry it shows a lower capacity factor. The resource was there; the grid was the constraint. That distinction is central to Pakistan's wind corridor problem.

Economic dispatch. A gas plant running rarely because it is expensive has a low capacity factor by choice, not by limitation.

Degradation. Capacity factor declines gradually over a project's life as equipment ages, which is why year-one figures are not lifetime figures.

Definitional differences. Capacity factor can be measured against DC or AC capacity, which matters for solar plants where the DC array is deliberately oversized relative to the inverter. The same plant can be quoted at different capacity factors depending on the denominator.

That last point is worth watching in any comparison. Ask which capacity the figure is measured against.

The connection to cost

Capacity factor feeds directly into levelised cost of energy. A plant's capital cost is spread across the energy it produces — so a higher capacity factor spreads the same cost across more megawatt-hours and lowers the cost per unit.

This is why an expensive offshore wind project can be competitive against cheaper onshore capacity: it produces considerably more from the same nameplate rating. We cover the calculation in our explainer on levelised cost of energy.

The bottom line

Capacity factor turns nameplate megawatts into meaningful energy figures, and it exposes the gap between announced capacity and actual contribution. Use it to compare technologies honestly — then remember it measures volume, not value, and that timing is often worth more than quantity.

Read the numbers behind the announcements

Capacity announcements make headlines. Capacity factors determine what they actually mean.

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