Our rule is pretty simple.
1) Elec when we have paid power, and outside temps are below 90 (we've had 113 days over 90 so far this year).
2) Propane when we have to pay for electricity.
3) Propane when we have 30 amp power.
4) Our water heater is under the kitchen counter. We have found that when we have typical normal Texas temps - over 90 often from early April to late October - having the water heater on raises the interior temperature. We have an external switch on the water heater to turn on electrical power, and an internal switch to turn on propane water heating. Much of the year, we will only turn on the propane switch when we need hot water.
We have water warm enough to shower, wash dishes, in less than five minutes. Then we turn off the water heating.