Intermatic WH21 electric timer
Bought electric timer, Intermatic WH21 model, for 40-gallon AO Smith electric water heater and this is my experience so far... after one year using for water heating in residential home.
My electrical bill went down by over 20%. Water heater works few hours per day only. It will be ON between 5 and 7 am and 6-8 pm and hot water temperature is around 120 F. As you can see, the timer is set in such a manner so my AO Smith water heater doesn't have to work constantly 24/7.
Very easy installation, it takes less than an hour, but you need some electrical experience.
The timer is very convenient. Intermatic WH21 model has many settings which covers almost any time you desire.
Instructions to install are poorly done but if you know what are you doing it is not hard to install. The only problem is the tight casing that makes very difficult to install #10 wire.