Backup power for a well pump
A well pump needs far more power to start than to run: Well pump 1/2 HP runs on 880 W and needs 2000 W to start. Size the backup source for the starting watts, then check the voltage.
Numbers updated from the published specs.
Running and starting watts
| Pump | Running | Starting | Start ÷ run |
|---|---|---|---|
| Well pump 1/2 HP | 880 W | 2,000 W | 2.3× |
| Well pump 1 HP | 1,500 W | 4,000 W | 2.7× |
From the calculator's appliance data. Your pump's nameplate or manual is the final word.
An induction motor draws a current spike for a fraction of a second while it accelerates. A generator or inverter must supply that spike, or it trips or shuts down. The battery capacity does not help here; the inverter's power rating does.
Which published products can start each pump
We compared the largest output each maker lists (surge, else rated or continuous) with the starting watts above. Only products that list a figure are counted.
| Category | Start 2,000 W | Start 4,000 W | Run 880 W / 1,500 W |
|---|---|---|---|
| Power stations | 101 of 179 (56%) | 46 of 179 (26%) | 65% / 58% |
| Portable and inverter generators | 370 of 380 (97%) | 278 of 380 (73%) | 100% / 97% |
| Home batteries | 28 of 28 (100%) | 26 of 28 (93%) | n/a |
Counts reflect what is in our catalogue, not the whole market. A listed surge is a short-term rating; the pump may also have a longer start time.
The 240 V caveat
Many well pumps run on 240 V, others on 120 V. Our catalogue does not record output voltage per product, so the shares above do not account for it. Watts alone are not enough. A pump that needs 240 V cannot run from a 120 V outlet. Read the pump's nameplate for voltage and amps, then the backup device's spec page for its outlets. Outlet voltages differ by model, so confirm 240 V outlets on the spec page. Amps equal watts divided by volts, so the same 880 W is 7.3 A at 120 V and 3.7 A at 240 V.
Energy for a day of water
The pump runs only when pressure falls. At 1 hour a day, the Well pump 1/2 HP uses 880 Wh. For a battery, the calculator's rule asks for about 1,334 Wh of capacity for that hour. The larger pump uses 1,500 Wh, needing about 2,273 Wh. If you run it for several hours, scale both numbers. Add the fridge and lights by using the calculator.
With a generator, the calculator's fuel rule applies at about half load. For a rough view, running the Well pump 1/2 HP alone is 0.35 L (0.09 gal) of fuel per hour of pumping. Real engines vary, so read the fuel use in the manual.
Practical steps
1. Read the pump nameplate for voltage, amps and horsepower. 2. Find the starting watts from the manual, or use the table above as a guide. 3. Pick a device whose surge rating exceeds that figure. 4. If the pump is hardwired, use a transfer switch or interlock, set up by a licensed electrician. See transfer switch vs interlock. 5. Run cables sized for the amps; see the wire size calculator.
To compare device types for a longer outage, read generator vs battery backup. Browse power stations, inverter generators and standby generators.
FAQ
Why does a well pump need more power to start?
An electric motor draws a short spike while it speeds up. In our data the Well pump 1/2 HP runs on 880 W but needs 2,000 W to start, 2.3 times more.
Can a power station run a well pump?
Some can. Of 179 published stations with a surge or output rating, 101 list at least 2000 W. Check the pump's voltage as well: many wells use 240 V, and many stations output only 120 V.
Does the pump run all day?
No. A pump runs when pressure falls, so run time depends on water use and on tank size. The calculator uses an hours-per-day value you can change.
Is a standby or portable generator better for a well?
A generator suits a pump that must run for days, and a 240 V pump is easier to feed from a generator with a 240 V outlet. Wire it through a transfer switch, not a wall outlet.