2.2 Efficiency and capacity factor
When energy is converted from one to another, what comes out is never as much as what goes in. The ratio (usually expressed as a percentage) is called the efficiency of the process:
percentage efficiency = (energy output/energy input) x100
Some types of energy conversion can be really efficient, for example up to 90% in a water turbine; others very low - around 10-20% in a typical internal combustion engine.
If you’re trying to assess an energy generator’s productivity in practice, one useful measure is its capacity factor (CF):
capacity factor = actual energy output over time / maximum possible output
The period of time can be a year, a month, a week or an hour. The units for the output quantities can be kWh, MWh, GWh, etc., and the capacity factor can be expressed as either a fraction or a percentage. The time period to which the capacity factor relates (a year, a month, a week etc.) should be stated.
Have a look at a couple of examples:
What would be the annual capacity factor of a 1 MW plant running constantly at a full rated capacity for one year?
One year = 365 days x 24 hours = 8760 hours in a year or so, the annual capacity factor = 8760 MWh / 8760 MWh = 1 or 100%
A 1 MW wind turbine generated 3000 MWh last year, so what would its annual capacity factor be?
We have already worked out that there are 8760 hours in a year, so: annual capacity factor = 3000 MWh / 8760 MWh = 0.342 or 34.2%