The term "installed capacity" refers to the amount of power an industrial wind facility would produce under ideal wind conditions; but such ideal wind conditions rarely (if ever) occur.
Wind Farm Proposal and Permits usually refer to the ratio of actual power to be produced compared to this production ideal as "an expected capacity factor of about 36% ". This is usually coupled with pie-in-the-sky expectations as: "north of 50% capacity factor" with the newer, larger turbines, new designs and technology. Yea, sure.
Let's look at the reality: according to FERC, for the whole of 2012 Maine's existing wind farms' actual power production was 24.72% of installed capacity, or a capacity factor .2472. That's less than one-quarter of installed capacity!
The turbines at Mars Hill, Rollins, and Stetson I and II are 1.5 MW GE machines. Do we have capacity factor figures for the newer, larger turbines? Well, where are they? Typically, the data will be available from FERC about two years after the newer larger turbines are installed in Oakfield.
So, when a wind promoter talks about "enough power for 20,000 homes", divide that figure by four, and you know the reality. I have yet to see any hard figures about the actual power production and capacity factor of the newer, larger turbines.