What Size Generator for a Well Pump?
A 5,000W portable with a 240V outlet is the practical answer for a 1 HP well pump (2,000 running watts, 4,000 at start). The watt math alone says a 3,500W class squeaks by, but well pumps are 240 volts, and 240V outlets effectively start at the 5,000W class. That voltage detail decides this purchase, not the wattage.
Wattage by type
| Well pump type | Running | Starting |
|---|---|---|
| 1/2 HP (shallow well) | 1,000 W | 2,100 W |
| 3/4 HP | 1,500 W | 3,000 W |
| 1 HP (planning number) | 2,000 W | 4,000 W |
| 1.5 HP | 2,500 W | 5,000 W |
| 2 HP (deep submersible) | 3,800 W | 7,600 W |
What it takes with company
Same engine as the wattage picker: running total plus the single biggest start spike, then the smallest class that carries it with 20% headroom.
| Load list | Running | Peak | Buy |
|---|---|---|---|
| Well pump alone (1 HP) | 2,000 W | 4,000 W | 3,500W portable |
| Well + fridge + the basics | 2,900 W | 4,900 W | 5,000W portable |
| Well + fridge + furnace + the basics | 3,700 W | 5,700 W | 5,000W portable |
The details that change the answer
Two things make well pumps the awkward case. First, they're 240V, so the generator needs an L14-30 or similar 240V receptacle, which inverter suitcases don't have; in the big-box lineup that outlet appears on 5,000W open-frame units and up. Second, they're hardwired through a pressure switch, not plugged in, so powering one means either a transfer switch/interlock at the panel or an electrician-installed inlet ahead of the pressure switch. Budget the wiring with the generator.
Depth drives the surge. A submersible starting under 200 feet of head works harder than the chart's bench numbers, and old motors with tired start capacitors spike higher still. If your pump is 1.5 HP or the well is deep, the 6,500W class buys real margin for a couple hundred dollars, which is cheaper than discovering the margin problem with a dry pressure tank in January.
Tactically: a pressure tank holds 5 to 10 gallons of usable water. Run the generator, refill the tank and every pot in the kitchen, shut it down. A well pump doesn't need continuous power; it needs scheduled visits.
Questions people ask
Can an inverter generator run a well pump?
Usually no, for the outlet rather than the watts: most inverters are 120V only, and well pumps are almost all 240V. The exceptions are large 240V-capable inverters and paralleled pairs with a 240V combining kit, which cost more than an open-frame 6,500W unit. Check the pump voltage on the pressure-switch wiring before shopping.
Why does my generator trip when the well pump starts?
The start surge is outrunning the peak rating. A 1 HP pump wants about 4,000W for the first second, and worn start capacitors push that higher. If a 5,000W-class unit (6,250W peak) trips on a 1 HP pump, suspect the capacitor or a long undersized wire run before blaming the generator; both stretch the start event past what the spec assumes.