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1) The Energy required to move any given volume of water to ground level can be calculated using M*G*H .... where H = the distance from ground level to the water level, in this case ~50ft....

2) The Energy required to elevate the same volume of water to a pressure of 65psi (@ ground level) is equivalent to the energy required to lift that volume of water to 65psi/.433psi per foot ==> ~150ft

3) Using 1 & 2 ==> To move 20 gallons of water from 50ft below ground to ground level at a pressure of 65psi is equivalent to moving 20 gallons of water to a height of 200ft (~61M).... 20 gallons of water weighs ~76kg; so the energy required = 76kg * 9.8m/s^2 * 61M = ~45,432J ... Assuming a flow rate of 20 Gallons Per Minute, this would imply 45,432J/60s = ~757W or Slightly more than 1HP

Obviously there are many mitigating factors that could influence the actual size of the pump required, BUT 757W is the actual work required to do the job.... Pump Efficiency and other variables would likely suggest sizing the pump larger perhaps 1.5HP to 2HP...., BUT the general method used to derive the amount of work requisite is sound?

Why am I using physics to figure this out? Major US MFGs offer 4in submersible well pumps from 1/2HP to 5HP .... AND each Power Rating is offered in various "Gallon Per Minute" models with little or no explanation as to the selection of one over the other ... Obviously a 1/2HP pump rated for a constant flow of 25 GPM can only do so with a very nominal amount of "head" ( < 80ft total ) .... while a 1/2HP pump rated at 10 GPM could operate nominally at up to ~200ft of total head. The fact that several distinctly different "GPM" rated pumps are available at any given power rating suggests that various pump models are optimized for use at various depths // output pressures AND Flow Rates .... For any given depth//pressure there is likely only one or perhaps two choices for nominal flow rate at peak efficiency ... At least that is my take on it .... But I just wanted to make sure I was figuring everything correctly ...

Thanks in Advance!

Fish