What sets the number
Three things, and only the first is on the spec sheet people look at.
1. Horsepower. Residential submersible pumps are commonly 1/2, 3/4 or 1 HP. Each step up is a substantial increase in both running and starting draw.
2. Depth and lift. A pump pushing water up 250 feet works harder than the same pump at 80 feet. Depth is why two houses with identical pumps can have different real-world draw.
3. Pump type. A shallow-well jet pump sitting in the basement behaves differently from a submersible hanging down the casing, and convertible jet pumps differ again.
Typical figures
| Pump | Running watts | What to plan for |
|---|
| 1/2 HP jet, shallow well | About 800-1,000 W | 5,000W class covers it with the rest of the list |
|---|
| 1/2 HP submersible | About 1,000 W | 5,000-7,500W class |
|---|
| 1 HP submersible | About 2,000 W | 7,500W class or above |
|---|
| 1.5 HP or a deep lift | Higher again | 9,500W and up |
|---|
Running figures are typical residential ranges. Starting draw is several times these numbers for a fraction of a second and is what the equipment has to absorb — the plate on the pump or its control box is the authoritative source for your own installation.
The energy is trivial. The power is not.
This is the surprise, and it changes what you buy.
A well pump only runs to refill the pressure tank. A household using around 200 gallons a day, with a pump moving roughly 10 gallons a minute, is asking the pump to run about 20 minutes across the whole day.
| Pump | 20 min of running | Share of a 2,000 Wh battery |
|---|
| 1/2 HP at 1,000 W | About 0.33 kWh | Under a fifth |
|---|
| 1 HP at 2,000 W | About 0.67 kWh | About a third |
|---|
Calculated as running watts x 20/60 minutes. Real household water use varies widely; conservation during an outage cuts this further.
So a day of normal water use costs a fraction of a kilowatt-hour. The battery or generator is not sized for that — it is sized entirely for the instant the motor starts. The starting watts page explains the physics.
Why 240V changes the shortlist before wattage does
A residential well pump is almost always 240V, wired to a dedicated circuit through a control box. Two consequences that eliminate most equipment immediately.
It needs a 240V source. A generator whose panel shows only 120V household sockets and a TT-30 RV outlet cannot run one at any wattage. Look for an L14-30R or a 14-50R first.
It needs a transfer switch or an interlock. There is no plug to use. The comparison page covers the two legal routes and what each costs.
Reducing what the pump has to do
Two moves that make a smaller machine sufficient.
Fill containers while the pump is available. Run the generator in blocks and fill jugs, buckets and the bath. The pump then does not need to be on the same schedule as your water use — the water storage page covers what is worth storing.
Ask about a soft starter. Some pump control boxes can be fitted with a soft start that ramps the motor rather than slamming it on, which substantially reduces the surge. That is a well contractor's job, and it can be cheaper than the next size up of generator.
The short version
- 1/2 HP is about 1,000W running; 1 HP is about 2,000W.
- Starting draw is several times that, and it is what you buy against.
- Energy use is tiny — under a kilowatt-hour for a normal day.
- 240V and a transfer switch come before wattage.
