Here are a couple of pictures of the generation mix on the electric grids from two very different locations. These times were not chosen for any particular reason except that your editor was just now looking at them and though he/she might write about them. First, 24 hours from Spain:
What is notable is that only 3 sources vary substantially: wind, solar, and hydro. And gas use goes up a bit for the midday and evening peaks. As wind rises early in the morning and does not decrease after the midday peak, it is clear that hydro is the source being adjusted to accommodate it. It is also clear that there is substantial hydro capacity that is drawn on for the evening peak (when there is no solar). One might therefore conclude that the large amount of wind power installed in Spain has served more to reduce the use of hydropower rather than fossil fuels.
Second, a week from the Pacific Northwest in the USA:
What is notable here is the effect on other sources of the steady rise of wind-powered generation over the past 3 days. It's harder to read than the Spanish graph, but the effect seems to be almost none.
Update, Dec. 4, 2013:
wind power, wind energy, environment, environmentalism