What size generator is needed to power (if need be) a 3 h.p. well pump, a 1/2 h.p. cistern pump, two 16 cu ft upright freezers, and a 16 cu ft fridge in the workshop?

  • Ok, what's the total rate wattage of those items? (Remember to allow for starting load, not just running load.) Personally I'd add at least a 10% buffer to that number.
    – keshlam
    Jun 12, 2016 at 23:12
  • These are all low-duty-cycle loads. If you can somehow prevent them from (once in a while) turning all on the same time, you'd be able to use a far smaller generator. Jun 13, 2016 at 0:29
  • 4
    It could be smaller, but it could not be "Far Smaller" and start a 3 HP well pump, which is the gorilla among the listed loads here. More details about that load (such as the motor's NEMA code, if it is a North American motor and has such a code) (and whether the user will load-shed before starting it, or not) would be required to provide any more specific answer. If the 3 HP pump fills a cistern that the 1/2 hp pump provides water from for use, load shedding would be very practical to set up.
    – Ecnerwal
    Jun 13, 2016 at 1:32

1 Answer 1


The 3 hp pump full load amperage should be 230 V 3910 VA x 125% since this is the largest motor 4887.5 VA.

The 1/2 HP would normally be 115 V 1176 VA.

3 refrigerators / freezer total 4320 VA for a total of 10,383 VA. I may be a bit over on the fridge/freezer size since I used 80% of 15 amp x 3.

I would not go lower than a system that had a peak rating of 10 kW but a continuous wattage closer to 7.5 kW should work.

Watch out. Some generators list their peak rated value and some list continuous rating trying to make a smaller unit look like a deal.

I have a 8.5/13? (peak/continuous kW) that runs 2 wells (3 hp & 1.5 hp both 230 V) 2 fridge/freezers, all the lighting in the house and barn and power to run the wood stove fan with some reserve.

  • One should also note that propane conversions on a liquid fuel generator results in a reduction of power. I've read that there is a 10 percent reduction due to the lower BTU capacity of propane, although one marketer suggests in the ad copy that they "counteract" that with some form of magic, I suppose.
    – fred_dot_u
    Sep 12, 2016 at 18:47

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