The capacity factor explains the link between a wind farm's nameplate power and its real output. How is it calculated and what drives it?
A wind farm with 100 MW of installed capacity does not generate 100 MW every hour. The wind blows, stops and strengthens. The capacity factor summarises that variability in a single number.
Definition
The capacity factor is the energy a plant generates over a period divided by the energy it would have generated running at full power throughout that period. For example, if a 100 MW farm generates 324 GWh in a year: 100 MW x 8,760 hours = 876 GWh theoretical output; 324 / 876 = 37%.
What affects it?
- Average wind speed: Power is proportional to the cube of speed; a 10% increase in speed raises power by about 33%.
- Hub height: Wind is steadier and faster higher up. A 120 m tower instead of 80 m can add two to four points.
- Rotor diameter: At low-wind sites, a large rotor lets the turbine run at high output for longer relative to its generator rating.
- Availability: A turbine under maintenance or fault does not generate; 94% instead of 98% availability costs about 1.5 points of capacity factor.
- Losses: Wake effects between turbines, electrical losses, icing and grid curtailment.
Why does it matter so much?
Picture two 100 MW farms, one at 40% and the other at 33%. The difference is about 61 GWh a year. At TRY 2,500 revenue per MWh that is about TRY 153 million a year. Over a 25-year life this gap decides the investment return.
Capacity factor and CO₂
Avoided CO₂ is directly proportional to generation. You can try your own scenario in the CO₂ Savings Calculator on this site.
A caution
The capacity factor may be quoted as gross (turbine output) or net (delivered to the grid). Make sure you use the same definition when comparing numbers. Also, as new-generation turbines raise capacity factors and cut cost per unit of generator capacity, older wind farms are being updated through repowering.
A small worked example
At Karataş the projected capacity factor with 5 MW turbines is 43%. Annual output per turbine: 5 MW x 8,760 hours x 0.43 = about 18.8 GWh. The 24 turbines generate 452 GWh in total. If older 3 MW machines were used at the same site, the capacity factor would stay near 35% and total output would be some 100 GWh lower. That is exactly how technology shows up in the numbers.
Comparison with industry averages
Across Turkey the average capacity factor of wind farms is around 33-36%. Our portfolio's weighted average of 36.7% results from careful site selection and high availability. In new projects we target above 40%.