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.
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 probability distribution, characterised by scale parameter c and shape parameter k. For speeds between cut-in and rated speed, power output scales approximately as a cubic function of wind speed relative to the cut-in threshold. Above rated speed, the turbine produces constant rated power until the cut-out speed, where it shuts down for protection. The calculator blends these regimes — partial-load cubic ramp, full-load plateau, and zero output outside the operating window — weighted by the Weibull probability of each wind speed occurring. Typical onshore capacity factors range from 25–40%; offshore from 40–60%.
How to use
Consider a 2,000 kW turbine with cut-in = 3 m/s, rated speed = 12 m/s, cut-out = 25 m/s, site average wind speed = 8 m/s, and Weibull shape k = 2. The calculator checks that 8 m/s falls between cut-in and rated speed, then computes the partial-load fraction: (8 − 3)³ / (12 − 3)³ = 125 / 729 ≈ 0.171. It then weights this by the Weibull probability for that speed bin. Summing across all speed bins yields a capacity factor — in this example approximately 32%. Annual Energy Production = 0.32 × 2,000 kW × 8,760 h ≈ 5,606,400 kWh (5.6 GWh) per year.
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.