Capacity factor is a plant’s actual electricity output over a period divided by the output it would have produced if it had run at full rated power for that entire period. UK government statistics call the same measure the load factor.
How to calculate it
Capacity factor = energy produced ÷ (rated capacity × hours in the period)
A 1 MW wind turbine running flat out for a year (8,760 hours) would produce 8,760 MWh. If it actually produced 3,066 MWh, its capacity factor is 3,066 ÷ 8,760 = 35%.
Why technologies differ
- Solar only generates in daylight and produces less in winter and under cloud, so its capacity factor in the UK is low compared with sunnier countries.
- Wind depends on wind speed. Offshore sites are windier and steadier than most onshore sites, so offshore turbines usually score higher.
- Nuclear plants run continuously except during maintenance and refuelling, so they have some of the highest capacity factors of any technology.
A low capacity factor does not mean a technology is inefficient. It describes how often the resource is available. That is why grid planners compare technologies on cost per unit of energy produced, not on installed capacity alone.
The UK’s Department for Energy Security and Net Zero publishes load factors by technology every year in the Digest of UK Energy Statistics (DUKES).