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# 1.2.2 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:

## Activity 1

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%

## Activity 2

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%

Let’s now look at the source of almost all of the earth’s renewable energy - the Sun

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