So far so good, but here comes the real challenge: Wind is intermittent. Sometimes it blows, sometimes it doesn't, and it's hard to predict more than a few hours in advance what it will do. Because of this, wind farms (groups of wind turbines) usually have two power ratings: A capacity number, and a capacity factor number.
For example, a wind farm might contain 200 wind turbines that are each rated at 1.5-megawatt. The capacity of this wind farm is 300 megawatts (200 x 1.5), but how much electricity it will actually produce depends on many factors, and if you look at the average production of all those wind turbines over a certain period of time - usually a year - and you divide that number by the maximum capacity of all those wind turbines, you get the capacity factor number.
So for example, if our wind farm above is operating at 30% capacity, it would be producing an average of 100 megawatts at any time. But this doesn't mean that you can count on 100 megawatts coming out; on some day it might be 300, and on others it might be 30. This is a problem not only because you need to build many more wind turbines than the capacity numbers might lead you to believe (and the media usually reports capacity numbers, not capacity factors), but also because of something else that we will see in the next section below.