If your home runs on well water, you're completely dependent on electricity for every glass of water, every flush, and every shower. This guide walks through exactly what size backup power you need, why well pumps are harder to size for than most appliances, and which type of backup makes sense for your situation.
Why Well Pumps Are Different to Size For
Most generator sizing guides tell you to add up your running watts. That works fine for lights and a refrigerator, but well pumps have a hidden catch: starting (surge) watts can be 3–4× higher than running watts for a second or two while the motor spins up. Undersize for that surge and the generator will stall or trip every time the pump kicks on — even if it looks "big enough" on paper.
Rule of thumb: Take your pump's running watts, multiply by 3.5, and make sure the generator's peak/surge rating clears that number — not just the continuous rating.
Well Pump Wattage & Generator Sizing Chart
| Pump Size | Typical Running Watts | Typical Surge Watts | Minimum Generator Surge Rating |
|---|---|---|---|
| 1/3 HP shallow well/jet pump | 500–750W | 1,500–2,250W | 2,200–2,500W |
| 1/2 HP submersible | 750–1,000W | 2,250–3,500W | 3,500–4,000W |
| 3/4 HP submersible | 1,000–1,500W | 3,000–5,000W | 5,000–5,500W |
| 1 HP submersible (deep well) | 1,500–2,000W | 4,500–7,000W | 7,000–8,000W |
| 1.5–2 HP submersible | 2,000–3,000W | 6,000–10,000W | 9,500–12,500W |
These figures are typical ranges — your pump's exact draw is on its nameplate or control box. If you can't find it, a clamp meter on the wire while the pump is running (or your well contractor's paperwork) will confirm actual running amps; multiply by 240V to get watts.
What Else to Add On Top of the Pump
Sizing for the pump alone is the minimum, not the plan. For a real outage, most households also want:
- Refrigerator/freezer: 600–800W running, 1,200–2,200W surge
- Furnace blower or well pump for a boiler system: 500–1,000W
- Basic lighting and a router/phone charging: 100–300W
- Septic effluent pump, if you have a pump-up septic system: similar surge behavior to a well pump — see our septic system guides for pump specs
Add these to your pump's running watts for a realistic total, and always size the generator's surge capacity around whichever single device has the highest surge draw (usually the pump), not the sum of everyone's surge at once — appliances rarely all start simultaneously.
Portable Generator vs. Standby vs. Solar Power Station
Portable (gas/dual-fuel) generator
The most affordable way to guarantee enough surge capacity for a well pump. Requires you to manually start it, run a cord, and refuel every 8–12 hours. Best for occasional outages and budget-conscious households.
Standby (permanently installed) generator
Starts automatically within seconds of an outage, runs on natural gas or a large propane tank, and needs no manual intervention. Highest cost and requires professional installation, but the least effort during an actual emergency.
Solar/battery power station
No fumes, no fuel runs, and safe to use indoors or in a garage. The tradeoff is runtime — a battery bank will comfortably handle a well pump's surge, but total watt-hours limit how many pump cycles you get before it needs recharging (solar panels or wall power). Good fit if outages in your area are usually short, or as a quiet, no-maintenance backup alongside a fuel generator for longer outages.
Transfer Switches: Don't Skip This Part
Never run an extension cord into your house or "backfeed" through a wall outlet — it's a serious fire and electrocution risk, and it can send power back down the line and injure utility workers. A transfer switch (manual or automatic) safely isolates your home's circuits from the grid before connecting generator power. Most well pump owners install a switch covering just the well pump, refrigerator, and a few lighting circuits rather than the whole panel — it's cheaper and matches what a portable generator can actually support.
Safety: Well pumps are almost always 240V. Confirm your generator provides genuine 240V output (not just two 120V legs that don't combine) before buying, and always have transfer switch wiring installed or checked by a licensed electrician.
Not sure what your pressure tank needs alongside backup power?
A correctly sized pressure tank means your pump cycles less often — which stretches every generator run further. Check yours free in 2 minutes.
Use the Free Calculator →Backup Power Options Worth Considering
9,500 running / 12,500 peak watts on gasoline — enough surge headroom for most 1–1.5 HP well pumps plus a refrigerator and lights. Runs on gas or propane, transfer-switch ready.
Check Price on Amazon →30-amp, 6-circuit manual transfer switch kit — the standard, well-reviewed way to safely connect a portable generator to your well pump and a few essential circuits. Professional installation recommended.
Check Price on Amazon →2,042Wh LiFePO4 battery, 3,000W AC output, no fumes or fuel runs — safe indoors. Best for shorter outages or as a quiet backup for a 1/2–3/4 HP pump; expandable with add-on battery packs for longer runtime.
Check Price on Amazon →Quick Answers
Will a 5,000-watt generator run my well pump?
Usually yes for a 1/2 HP pump, and often for a 3/4 HP pump — but check the surge rating specifically, not just the running-watt number, since that's where undersized generators fail.
Can I just plug my well pump into the generator with an extension cord?
Only if your pump is a 120V surface pump wired to a standard outlet — most 240V submersible pumps are hardwired and need a transfer switch, not a cord.
How long can a portable generator run a well pump?
Pumps only run in short bursts to refill the pressure tank, so fuel consumption is much lower than running a generator continuously — a 6-gallon tank often lasts 8–12+ hours even with the pump, fridge, and lights included.