Wind Turbine Capacity Factor Calculator
Estimate a wind turbine's capacity factor and annual energy output given its power curve and site wind distribution. Essential for wind farm feasibility studies and investment analysis.
Last updated: September 2026
Formula below · 2 sources (energy.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Capacity factor (CF) is the ratio of actual annual energy output to the theoretical maximum if the turbine ran at rated power 100% of the time: CF = AEP / (Rated Power × 8,760 hours). Real wind speed follows a Weibull distribution with shape k and scale c = mean / Γ(1 + 1/k). The calculator uses an idealized power curve: zero below cut-in, (v³ − cut-in³) / (rated³ − cut-in³) between cut-in and rated speed (power rising with the cube of wind speed), full rated power from rated speed to cut-out, and zero above cut-out. It sums powerFraction(v) × WeibullDensity(v) in 0.05 m/s steps. Real power curves, availability, wake and electrical losses typically reduce the result by a further 10–20%. Typical onshore capacity factors range from 25–40%; offshore from 40–60%.
How to use
Consider a turbine with cut-in = 3 m/s, rated speed = 12 m/s, cut-out = 25 m/s, a site average wind speed = 8 m/s, and a Weibull shape k = 2. The scale parameter is c = 8 / Γ(1.5) = 8 / 0.8862 ≈ 9.03 m/s. The power fraction at the mean speed alone would be (8³ − 3³) / (12³ − 3³) ≈ 0.285, but the distribution also delivers many hours at or above rated speed; weighting every 0.05 m/s bin by its Weibull probability gives a capacity factor of about 37.8%. With k = 3 (steadier wind) it is about 36.9%. For context, a 2 MW turbine at 37.8% would produce about 0.378 × 2,000 kW × 8,760 h ≈ 6.6 GWh per year before losses.
Frequently asked questions
What is a good capacity factor for a wind turbine?
A capacity factor above 35% is generally considered good for an onshore wind turbine; below 25% often indicates a poor wind resource or an oversized generator for the site. Offshore turbines regularly achieve 45–55% due to stronger, more consistent winds. Modern onshore turbines optimised for low-wind sites (large rotor, small generator) can reach 40%+ even at modest wind speeds. Capacity factor is a key metric used by lenders and investors to compare the economic performance of wind projects.
How does the Weibull shape parameter affect wind turbine energy production?
The Weibull shape parameter k describes how variable the wind speed is at a site. A value of k = 2 (Rayleigh distribution) is a common assumption for temperate locations and indicates moderate variability. Higher k values (e.g., k = 3–4) mean wind speeds cluster tightly around the mean, which can increase capacity factor if the mean speed is near the turbine's rated speed. Lower k values (k < 1.5) indicate highly variable winds with many calm periods, which reduces capacity factor. Getting k wrong by even 0.5 can shift annual energy estimates by 5–10%.
Why do wind turbines have a cut-out speed and what happens when wind exceeds it?
Cut-out speed (typically 20–25 m/s) is the upper limit beyond which the turbine shuts down to prevent structural damage from extreme aerodynamic loads. When wind speed reaches this threshold, the blades are feathered (pitched parallel to the wind) and the rotor is braked to a stop. Some modern turbines use 'storm control' algorithms that allow continued operation at reduced power above cut-out to avoid abrupt grid disconnection events. The gap between rated speed and cut-out represents only a few percent of annual hours at most sites, so the energy lost to cut-out events is usually small — but the structural engineering implications are enormous.