Imagine a town supplied by hydro. The dam produces enough for the whole town on the weekends, but only enough for the businesses during the work week. Lets say during the work week the homes are powered by coal instead, because it's imported cheap. The more energy they use during peak hours, the more pollution they're responsible for.
Very much simplified, but you can see how using energy during peak hours is bad...
In this case, every little bit of energy the homes use during a work-day is from a dirty source. If they shift all their loads to off-peak hours then it would be a green oasis. But, some people stay home doing laundry, vacuuming, etc...for us guys that's pressure washing the boat or making a tool to make a tool that makes a tool.
Pollution is bad by definition, whether you believe in global warming or not. 