I recent obtained my dads old place and since it sat for so long the generator was stolen and so was our water tank, and now I am in the process of replacing the large generator and I wanted to see how I can find out exactly how large of generator I will need. The generator he had was a 25kw 3 phase generator. The pump is a 10 HP that is fairly old and pumps 100 gallons minute. According to the Arizona Dept Water Resources my "Well Depth is 340ft, Water Level 250, and casing depth 200ft, and casing diameter is 8 inches. And I am completely off grid with no power. I guess I have a few questions.

What size generator do I need just for this pump?

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How can tell what voltage it is, I see generators of this size for sale that vary on voltage i.e. 208 240 120. I have pictures of the electrical box and how it is wired but I dont understand what it means.. enter image description here

Is there a better option for my type of well that would still allow me to pump 100 GPM? i.e. new generator and pump/motor combo?

Maybe these are questions are better answered in a electrical forum but I found this post and decided to give it a try.

  • Is that a submerged pump?
    – wallyk
    Jan 22, 2017 at 6:15
  • 3
    100GPM? Get a low-flow showerhead LOL... No seriously there's a whole bunch of the story that you're leaving out. What kind of distances are we talking between generator, home, and well site? Jan 22, 2017 at 16:37
  • You are off grid so I assume you're not watering five hundred head of cows.. Irrigating? Why would you possibly need a hundred gallons a minute from your well.. You know, normally it's around 10 to 12 gallons per minute from a one horsepower submersible, which is pretty much standard for farm use, with 200 head of cattle.
    – Reg Graham
    Jan 21 at 1:02

1 Answer 1


250 ft head for the well = about 125 psi. You typically run the local system at 60 psi, so 180-200 psi.


says 15 hp. If we use the usual 1000W power to produce 1 hp, that's 15 kW. Your Granddad's 25 kW generator wasn't overkill. (It's actually 745 w/hp, but you've got a bunch of inefficiencies nibbling at you too.)


Newer pumps are considerably more efficient. Gould has a line that has a built in VFD (variable frequency drive) You feed it single phase, it converts it to 3 phase, and also adusts the speed of the pump so that it doesn't cycle nearly as often. Do they have 3 phase VFD? Never looked.

If the place was set up and working with 3phase, then keep it. If you are going to do a lot of repairing/converting to solar to reduce the generator run time, etc, then getting a largish single phase generator, and converting where needed to 3 ph using VFDs may save you a lot of aggravation.

With an 8 inch casing, you may be able to run a windmill pump as well as your electric one at the same time. If the windmill fills a lined dugout, your power bill is cut in half.

Generators don't loaf well. You want to avoid having a generator under less than half load -- the engine runs too cool, and you are using too much fuel chasing bits of metal around in cicles.

Having a generator house with temperature controlled louvers, or exhaust fan will help keep the engine at it's best operating temperature.

If you are really concerned with efficiency, then have several units of various sizes, that cut in and out to match the demand. This is a difficult control set up, well beyond my expertise.

I would consider consulting a power engineer for this setup. If you have to replace the generator, and are thinking about the pump, then a few hundred bucks for a professional opinion may save you a lot of time and money.

Meanwhile, you can get a feel for how things are really operating by renting a 25 kW 3ph generator, and put a recorder to figure out how much of the time it's running, and how much power it's using. An engineer will want this information anyway.

Or Buy the generator at an equipment auction, if you need to resize later, sell it -- probably get what you paid for it.

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