The term Availability is used in the context of power generation to refer to the percentage of time that a power plant is available to generate (i.e. not shut down for maintenance, out of action because of breakdowns and so on). Obviously high availability is good, although it doesn’t in itself say anything about actual generation.

The metric used to reflect lower-than-theoretical-peak energy generation over time is Capacity Factor, which includes lost generation both from non-availability (plant out of action) and lost or lower generation for other reasons such as lack of wind.

For example, you’d expect an onshore wind turbine to be available say 98% of the time, but for a sizeable part of that time there could be limited (or no) wind. The overall capacity factor in a reasonable wind area might be 35%.

On the other hand there could be an offshore wind turbine with lower availability, let’s say 90%. That lower availability might not reflect more breakdowns, but simply longer breakdowns, since it takes more time to get out to an offshore platform to fix things. However when it is operating, the better wind resource offshore means it’s operating close to peak output more often. So overall capacity factor might be higher, say 45%.

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