[folivier]

Since we're full timing and currently in Louisiana and need air conditioning 24/7 here's what we do: First thing in the morning while coffee's brewing I turn on electric aqua hot for about 1/2 hour. That gives us enough hot water for 2 showers or whatever else all morning. Then at night either turn on electric or diesel if we take showers at night.
I find that once the temperature is 200 degrees or so then turn off the aqua hot we will have enough hot water for 2 good showers.
I treat the diesel aqua hot similar to the generator. Run at least once a month whether needed or not.
Now in winter or cooler climates if electricity is "free" we leave it on electric. If not then do the same as above.