Capacity Factor
Definition
Capacity factor is the electricity a generator actually produces over a period divided by the maximum it could produce if it ran at its full rated capacity for every hour of that period. A 100 MW plant that produces 219,000 MWh in a year has a capacity factor of 219,000 / (100 x 8,760) = 25%. It reflects the resource (sun, wind, water), outages, curtailment and, for dispatchable plants, how often the plant is economic to run.
Why it matters
Capacity factor is the link between a plant's rating in megawatts and the energy it sells in megawatt-hours. Offering documents often quote capacity in MW; revenue depends on MWh. Checking the assumed capacity factor against EIA averages for the technology, and the project's actual history against its forecast, is one of the quickest tests of a projection.
Common misconceptions
- •Capacity factor is not efficiency; it measures output against rating over time.
- •A plant's rating may be stated in AC or DC terms for solar; the same output gives different capacity factors on each basis.
- •Availability and capacity factor differ: a plant can be fully available and still produce little if the sun or wind is weak.
- •The label is only a starting point. Investor outcomes usually depend on the governing documents, cash-flow priority, valuation policy, disclosure quality, and manager discretion surrounding the term, not just the shorthand definition used in marketing materials.
Technical details
Formula
Capacity factor = actual energy produced (MWh) / (rated capacity (MW) x hours in the period).
A year has 8,760 hours (8,784 in a leap year).
Example: 50 MW wind farm, 148,000 MWh in a year = 148,000 / 438,000 = 33.8%.
U.S. utility-scale, 2025 (EIA)
Nuclear 91.1%; geothermal 65.9%; natural gas combined cycle 58.7%; coal 48.8%.
Hydroelectric 34.4%; wind 33.8%; solar photovoltaic 24.0%.
Natural gas combustion turbines 13.6%: peaking plants that run when prices are high.
Worked example: Clearway Energy, 2025 10-K
Solar weighted-average capacity factor 28.9% (29.8% in 2024); wind 30.6% (30.1% in 2024).
Flexible Generation equivalent availability factor 93.4%: a measure of availability, not output.
How investors apply the term
Investors usually apply this term by tying the definition back to the controlling documents and then asking which cash flows, rights, restrictions, or valuation assumptions it changes. A useful review looks at the term alongside fees, leverage, liquidity, reporting frequency, and manager discretion rather than treating it as a standalone concept.
The practical diligence test is simple: identify who benefits if the assumption is wrong, who has control when conditions change, and what evidence the investor receives after closing. Terms that cannot be reconciled to documents, cash movements, and monitoring data should be treated as marketing shorthand until proven otherwise.
Frequently asked questions
- What are typical capacity factors?
- EIA's figures for U.S. utility-scale generators in 2025: nuclear 91.1%, natural gas combined cycle 58.7%, coal 48.8%, wind 33.8%, hydroelectric 34.4%, solar photovoltaic 24.0% and natural gas combustion turbines 13.6% (Electric Power Monthly, Tables 6.07.A and 6.07.B, October 2026 release).
- Why does it matter to investors?
- Under an as-generated PPA, revenue is price times output, so a project whose capacity factor falls short of the forecast earns less even at the contract price. Clearway reported 2025 weighted-average capacity factors of 28.9% for its solar fleet and 30.6% for wind, noting that typical solar capacity factors are about 25% and wind 25% to 45%, varying with season and weather.
- Is a low capacity factor bad?
- Not necessarily. Peaking gas turbines run rarely by design and earn capacity payments for being available. For wind and solar, capacity factor depends mostly on the site and technology, and has to be compared with the forecast the investment was based on.