It's not quite as bad as that, since you don't consume fossil fuel resources when plants aren't running, but this is mostly a benefit for oil/gas plants, not coal (since the coal is likely useless without the power plant consumer anyway)

That might work in The SouthWest of the USA, but the Northern bits dont get enough sun to get through the day, so you'll need fossil fuel plants all day night for the Winter. If they're there already its cheaper to use all day in the summer as well.