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I'll give you a 'for instance'. For $150 I bought a Southern piston pump with 3/4 inch discharge. It took almost 2 hours of running that pump off my generator to fill a 200 gallon water tank.
Fed up, I bought a Honda water pump with 2 inch discharge and filled that same tank in 5 minutes.
You can also pump to a gravity tank above the heater, to give volume and pressure..
The pump feeds the tank which gravity feeds to the heater, when you turn on the water. When you turn off the water the pump will top off the tank for the next time.
That way you can still use water if the power goes off. At least till your tank empties.
I can't remember the formulas for calculating water pressure generated by height and reductions ( 2 in to 3/4 in for example ).