Well Pump Generator Size Calculator (by HP)
Data reviewed ·how we calculate
For a house on well water, the pump is the outage load that turns a power cut into a water cut — no flushing, no washing, no drinking supply — and it is also the sizing problem people get wrong most expensively. Two traps: submersible well pumps start hard, demanding two to three times their running watts, and nearly all of them run on 240V, which the small generators people already own simply cannot supply. Pick your pump’s horsepower below and the calculator handles both.
Size a generator for this load
Computed on typical values; ranges shown below. 25% headroom applied.
How to size it step by step
Find your pump’s horsepower: check the well installation paperwork, the pressure-switch/control-box label, or the county well record. If nothing is on file, a 2–3 bedroom home on a typical well is most often 1/2 HP.
Confirm the voltage — this decides the generator before any wattage does. Nearly all submersible well pumps are 240V, which requires a generator with a 240V outlet (L14-30R or similar); 120V-only inverters are out regardless of watts.
Size to the surge: a 1/2 HP pump runs at ~1,000W but starts at ~2,100W. With 25% headroom that is 2,625W of 240V-capable generator — in practice the 3,500–4,500W class, since 240V outlets start appearing there.
Plan the connection: well pumps are hardwired to the pressure switch circuit, so feeding one from a generator requires a transfer switch or interlock installed by a licensed electrician. Budget it with the generator.
Pro tips
- Check your nameplate first — every figure on this page is a planning estimate, and the label on your specific unit beats any chart.
- Fill bathtubs and jugs when a storm is forecast — stored water buys you the luxury of running the generator on your schedule instead of the pressure tank’s.
- A larger pressure tank (or correct precharge in the existing one) cuts pump starts dramatically — fewer surge events is the cheapest sizing upgrade a well system can get.
- Label the pump breaker in your panel now; during the outage you (or the electrician) will want to find it fast.
The data behind this calculator
| Figure | Value | Source |
|---|---|---|
| 1/2 HP running / starting | ~1,000W / ~2,100W | Generac sizing chart; Franklin Electric motor data |
| 3/4 HP running / starting | ~1,500W / ~3,000W | Generator sizing charts; Franklin Electric motor data |
| 1 HP running / starting | ~2,000W / ~4,000W | Generator sizing charts; Franklin Electric motor data |
| Voltage | 240V for nearly all submersibles (a few shallow-well jet pumps are 120V) | Franklin Electric / Goulds pump nameplate conventions |
Duty cycle: The pump runs only when the pressure tank calls for water — minutes per hour in normal use. But every pressure-tank cycle restarts the motor, so the generator must cover the full starting surge dozens of times a day. Deeper wells load the motor harder; expect the top of each range past ~300 ft.
Well pump generator questions, answered
What size generator do I need to run a 1/2 HP well pump?
About 1,000W running and 2,100W starting — with 25% headroom, 2,625W, which lands in the 3,500–4,500W portable class. In practice that class boundary is convenient: 3,500–4,500W generators are also where 240V outlets become standard, and 240V is non-negotiable for nearly all submersible pumps. Add the household basics (fridge, lights) and the same class still covers it — see the well pump scenario page for that combined math.
Why does my well pump need a 240-volt generator?
Because the pump motor itself is a 240V machine: submersible pumps run on both legs of your panel’s split-phase supply to keep amperage (and wire size down the well) manageable. A 120V-only generator cannot produce the second leg no matter its wattage. Look for a generator with an L14-30R (240V/30A) outlet, and have an electrician wire a transfer switch or interlock to the pump circuit — hardwired pumps have no plug to improvise with, and improvising is how backfeed accidents happen.
How much water do I actually get per hour of generator time?
More than you might think, thanks to the pressure tank. A 1/2 HP pump delivering ~10 gallons per minute refills a typical pressure tank in a couple of minutes and then rests. Run the generator for an hour and you can flush toilets, fill jugs and shower; the pump itself may only run 10–15 of those minutes. Many well households run the generator in 2–3 daily blocks and have effectively normal water.
Why does the generator handle my pump’s running load but stall when it starts?
The starting surge is doing exactly what the label warned: a submersible starting against the water column draws 2–3× running watts for a fraction of a second, and a generator without that surge margin sags or trips. Common aggravators: other loads running when the pressure switch clicks (stagger them), a long undersized cord run (voltage drop makes starts harder), or a worn pressure tank bladder causing rapid cycling. If the math should work but doesn’t, have a well tech check the tank precharge before buying a bigger generator.
Does well depth change the wattage numbers?
Meaningfully, yes. The horsepower classes here assume typical residential depths; the same 1/2 HP motor working a 400-foot column runs toward the top of its range, and very deep wells simply get bigger pumps (which is why confirming YOUR pump’s HP beats guessing from house size). The nameplate amps on the control box × 240V is the definitive running figure — our numbers are planning estimates, as everywhere on this site.
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