There's also a given that 6hrs after sunset there isn't a need for as much power... most of the population is asleep, lights off, TVs off, etc. There are other traditional power plants (Hydro and Nuclear) in AZ that provide overflow service for off hours. It actually takes a combination of plants to offer the best load distribution.

In parts of AZ there are serious brown outs and rolling blackouts in the summers as A/C kicks in everywhere, which is probably the biggest draw.

I'm opposed to coal altogether for pollution reasons alone. It's bad enough between the cars and airport traffic, don't need more.