Calculating annual power production

Depends what "mean" means to you.

To me it means that at any one instant, the power production may be more, or less, than 100Kw, but over the course of a year it averages out to 100Kw.

To arrive at the mean, you take x number of samples, add them together and divide that total by the number of samples taken. It's just the average.

100 x 365 x 24 = 876,000 kwh (in leap years add 2400).

I take that to read that the average amount of power generated at any one instant is 100kW, is that correct? (Don't know if this figure is single or three phase)

Yes, but if, for example you're talking about a solar plant, that average may come from something like 6 hours of 400 kW and 18 hours of 0 kW. (Similarly for windmills or anything else that may be based on a kinetic energy supply that varies over time.)

BTW, it doesn't matter whether it delivered as single or three phase (as long as the guy putting the data together hasn't calculated wrong).

If that is correct, what is the annual amount of power generated?

There are about 8000 hours in a year, so 8000 hours x 100 kWs average equals 800,000 kwhrs.

I'm now retired, if you think I'm getting off my a*se to generate my share you can think again!

I guess you might be a little "mean" then

Mean, I'm titter than a duck's a*se. That's water tight!