What is the availability of a power station?

It is the amount of hours a power plant is online, per year. It changes for each station, depending on their reliability.

Average is 80% for steam plants, 90% for internal combustion. Nuclear are very maintenance intensive and prone to trouble, therefore their average is only about 80% overall.

A plant can run a 100% of the time in one year, but it will have to be taken out of service shortly after for a significant period of time, for maintenance.

Hydro are the most reliable of all and can run for years without being taken out of service.

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Availability is the fraction of time that the station is available for service.

So, if there are 8760 hours in a year, the plant is available 8760 minus (forced downtime hours plus scheduled maintenance hours).

Hydro and nuclear plants have very high availability. Gas turbine plants have a slightly lower availability. Wind turbines have an availability that could be as low as 40% (mainly controlled by the availability of wind), etc. And the availability of individual plants varies from the average for classes of plants.

The availability of an individual plant might be considered by its owner to be confidential information because it has an impact on its market competitiveness.

It is very near 1, but I don't know an exact number. One reason it is so high is that they have on-line back ups (redundancy) for much of the equipment. Off line back ups can quickly be switched in.

You might be helped if you give more information of what you want, and what is required.