In reply to chaparral:
The problem is that the amount of reserve capacity you have to have up and running at any given time is dictated by the highest peak load you have in a certain area for the past year, which is a relatively fixed amount. It's set up to protect from a suddenly cold night, or high demand (like say sump pumps) catching the utility off guard and load having to be dropped.
In the pre-wind days you would take several units down from 80% to 70% or the like and the efficiency of the units would be relatively constant so the lost productivity was small effect in the long run. Now with the wind online, the same number of unit have to be up and running to have the required capacity, and on a windless night all is as it was. On a windy night, though, the wind is making power and those units have to come down in power for every MW the wind puts on the grid. There is always a minimum, though, and at some point you can't throttle down a unit without taking it off line.
The other really important facet of wind is that since speed can't be controlled they are usually inductive generators. Inductive generators are limited in the amount of reactive load they can supply, and they need another generator hooked to them to maintain voltage properly. The other generators being lightly loaded make them hard to control voltage wise, and they reach a limit where they can't maintain voltage either. So practically we are limited to just so much wind a a percentage of the whole.
If we could get a bit more load at night, though, we would be more stable and could use more of the wind during the day. Ideally we would add more pumping hydro, but try building a dam these days. Instead I say burn a bulb all night and do your welding at 3am. It'll make my job easier for sure.
