Sizing a generator for a well pump
Last updated
Well pump running and starting watts by horsepower, 120 V vs 240 V pumps, and how to find your pump's specs.
The USGS estimates that more than 43 million people in the United States get drinking water from private wells. When the power goes out, the pump stops, and so do the toilets, showers, and kitchen tap. For well owners, the pump is often the load that decides generator size.
Typical well pump wattage
Champion Power Equipment lists:
| Pump | Running watts | Starting watts |
|---|---|---|
| 1/2 HP well pump | 750–1,000 | 1,500–2,000 |
| 1 HP well pump | 1,000–2,000 | 2,000–4,000 |
Larger pumps (1.5 to 2 HP and up) draw more. Deep submersible pumps often have high starting current, and the surge is the number that stalls undersized generators.
120 V or 240 V?
Many deep-well submersible pumps run on 240 volts. That matters for two reasons. First, the generator must produce 240 V, which usually means a conventional generator of about 5,000 watts or more, or a large inverter model with 240 V output. Second, the pump is hard-wired, so it must be connected through a transfer switch or interlock, not an extension cord.
Find your pump’s real numbers
- Look at the pressure switch or control box (often in the basement or well house). The control box label lists horsepower, volts, and amps.
- The pump’s breaker size gives a clue; a two-pole breaker means 240 V.
- Your well driller’s records usually list pump model and horsepower.
Multiply the running amps by volts for running watts. If the label lists “service factor amps,” use that as a conservative figure.
Sizing example
A 1 HP 240 V submersible (2,000 running, 4,000 starting from the chart) plus a refrigerator (400), furnace fan (800), and lights and internet (250):
- Running total: 3,450 W
- Largest extra surge: the well pump, 2,000 W
- Peak: 5,450 W
- With 20% headroom on running: about 4,300 running watts
A generator of roughly 5,000 to 7,500 running watts with 240 V output and a starting rating well above 5,450 W gives comfortable margin. If the generator is just barely large enough, start the well pump before the refrigerator and other motors.
Save water, save fuel
The pump runs only when the pressure tank drops. Running the generator in blocks and avoiding laundry during outages reduces pump cycles. If you have a large pressure tank, you may get several flushes and a shower between pump runs.
Try your numbers in the generator size calculator.
More on private wells
For how the pump, pressure tank, and well fit together, see how private wells work. If a storm also flooded the wellhead, follow after a flood: your septic system and well before you drink the water.