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Maintaining a temperature is indeed easier, but it requires almost constant energy, versus a one-time energy expenditure to raise the temperature to the desired temp. But is that quasi-constant expenditure greater than that one-time expenditure? That's the question...
An interesting twist to the question is when you consider that there are tankless water heaters that supposedly use a lot less energy by only heating water when there's an actual demand for it. (It doesn't appear to be worth it to replace my hot water heater with one of these right now, but in 10-15 years when it's time to replace it anyway it's something to think about.) Wouldn't using a timer on a standard hot water heater be a kind of hybrid approach between it and a tankless one?
I may experiment with this some, by manually turning off/on the water heater and comparing gas usage (I can do it for a week, reading my own meter, rather than having to wait for the gas bill, since it's warm enough the furnace shouldn't really be using any gas right now), before I go out and buy a timer. Or, I might just buy the timer anyway, since there's no reason I couldn't use it for other things if it doesn't save me any money on the water heater.