Capacity Factor
The capacity factor of a power plant is the ratio of the actual output of a power plant over a period of time and its output if it had operated at full capacity over that period of time.
The actual output of the power plant over a period of time
The power plant’s output over a period of time if operating constantly at full capacity
An example:
If a 100 MW power plant operated at full capacity for a whole year it would produce 876,000MWh of energy.
No power plant operates at full capacity all of the time. If the 100 MW power plant actually produced 438,000MWh of energy over a year it would have a capacity factor of:
438,000 / 876,000 = 0.5 = 50%
The capacity factor is often quoted over a year and commonly used when describing the output of wind farms. A wind farm tends to have a capacity factor of approximately 30%, although there are wind farms in the UK with capacity factors in excess of 40% because of the good wind resource.
Conventional power plants are often assessed on a similar ratio called the ‘load factor’. The load factor is the ratio of the average load to the peak load during a period of time.
Average load over a period of time
Peak load during a period of time
The Digest of UK Energy Statistics 2005 shows that the average load factor of generation plants in the UK was 66.3% in 2004.
Some opponents of wind farms state that wind farms only generate energy for 30% of the time. This is wrong and results from misunderstanding of what 'capacity factor' means.
Why does a wind farm have a capacity factor of approximately 30%?
A wind turbine's output is dictated by the wind speed at a site. Every wind turbine has a power curve which illustrates how much energy it will generate at a given wind speed. A power curve is shown below:
As you can see the energy generated changes with wind speed and the wind turbine starts up at 4m/s and shuts down at 25m/s for safety reasons. If you consider that the likely average wind speed at the site is 7m/s it is possible to understand why a wind turbine can actually be generating electricity for a large proportion of the year (around 75% of the time) and still have a capacity factor of 30%.