They tend to exceed energry costs because energy costs are dominated by the cost of purchasing oil, which is as easy to store as water. We're moving away from that now, towards a world where solar power plants produce power at stunningly low prices some of the time (like €0.01114/kWh on the top google hit just now) and those who need a lot of power and can adapt their usage to match solar power plants can make use of those prices.