[Piwoslaw]

Newer hot water tanks have digital thermometers so that the water has a specified temperature at a specified time of day. The rest of the time it's cooler. There's usually lots of insulation, so it doesn't lose that much heat overnight, or between morning and evening. Also, if it's electric, you might be able to take advantage of cheaper electricity at night, warming water when it's cheaper and then "coasting" through the rest of the day. Also, tanks can take advange of other sources of hot water, like solar or heat pump, they can also be used to dump extra power from wind turbines when it's windy. If you're planning on not being home for longer periods, then you just turn it off.