The challange is to do it in your head:

So Bill Gates buys a 5 megawatt power plant, don't know what it is, 100 acres of PV, a nuke, whatever, it generates 5 megawatts. One of his uses is to heat up his double size swimming pool, 40,000 gallons. So the water started out at 40 degrees F (Brrrrr), and he wants it at a languid 90 degrees F. How long does it take the water heater being fed 5 megawatts, to heat the pool those 50 degrees? If you don't know, 1000 watts ~= 3200 BTU and a pint's a pound the world around.
The other thing is, he started a business selling the electricity he doesn't use to heat the pool. How much money a year does he make selling the power at 5 cents a KWH?