Well Pump Backup Power Calculator

The energy side of this uses the same method as the main Power Station Size Calculator. For a well pump, two more specifications decide whether a unit can back it up, and it has to pass all of them:

• Battery capacity, in watt-hours (Wh), is the stored energy. It sets how long the pump can keep cycling before the battery is empty.
• Inverter output, in watts (W), is how much power the unit can deliver at once: a continuous rating for the running load and a higher surge / peak rating for the spike when the motor starts.
• AC voltage — whether the unit outputs 120V, 240V, or both, in an outlet your pump can use. A 240V pump will not run from a 120V-only power station.

A large battery behind a small inverter will not run the pump; a powerful inverter with a small battery starts it but drains fast; and the right watts at the wrong voltage does not help at all. The calculator above reports the watt-hours and the watts separately, and flags voltage as a check you must do yourself. To see how long a given unit lasts against a steady load, use the Power Station Runtime Calculator.

Running watts is the steady power a well pump draws while its motor turns and it is pushing water. It determines energy use over an outage and the continuous output a power station must sustain.

Startup, starting, or surge watts is a brief spike — often a fraction of a second — as the motor overcomes inertia and the system builds pressure. On a well pump it can be substantially higher than the running figure. A power station's inverter has to deliver that spike or the pump will not start, regardless of how much battery is behind it.

How much higher the startup figure is depends on the motor type, the pump, the controller, and the supply voltage, so this calculator does not estimate it. Both numbers are usually on the pump's rating label, in its manual, or in the pressure-switch or constant-pressure controller documentation. The same distinction applies to a sump pump, which has its own calculator.

Residential well pumps come in both 120-volt and 240-volt versions. Shallow-well jet pumps and smaller pumps are often 120V; many deeper-well submersible pumps, and most higher horsepower pumps, are 240V. Check the rating label, the pressure switch, or the manual to find which one you have — it is not something you can assume from the horsepower.

This matters more than it does for a sump pump, because it is a hard limit:

• Many portable power stations provide 120V AC only.
• A 240V well pump cannot be powered directly by a 120V-only power station, no matter how large its battery or how high its 120V output rating.
• Running a 240V pump from a portable unit requires a power station that provides a true 240V output (some larger models do, sometimes by combining two units) with an outlet or connection your pump can use.

This page does not calculate transformers, split-phase wiring, or 120V-to-240V conversion. If your pump is 240V, confirm a candidate power station's 240V capability, outlet type, continuous watts, and surge rating before buying — watt-hours alone will not tell you whether it can run the pump.

Get the numbers from the source, not from a horsepower rule of thumb:

• The rating label on the pump or its motor. It may list watts, or amps and volts — watts is roughly amps multiplied by volts. Look for a separate starting, surge, or locked-rotor amps figure, and note the voltage.
• The owner's manual or the specifications on the manufacturer's product page, which often list running and starting wattage and the required voltage explicitly.
• The pressure-switch or controller documentation for constant-pressure or soft-start systems, which changes the startup behaviour.
• A plug-in or clamp watt meter where the pump is on a standard outlet. The steady reading is the running figure; a peak-hold feature captures an approximate surge, though a very brief inrush can be higher.

Horsepower — 1/2 HP, 3/4 HP, 1 HP, 1.5 HP — is a useful search term, but it does not give a reliable wattage. Two pumps with the same horsepower label can draw noticeably different running and starting watts depending on the motor, the pump design, the controller, and the supply voltage, so this calculator asks for the measured or spec-sheet numbers instead of converting from horsepower.

A well pump does not run continuously. It switches on when the pressure tank drops to the cut-in pressure, refills the tank in a minute or two, and switches off at cut-out. How much it runs over an hour depends on how much water the household uses — a few minutes with normal use, more with laundry, irrigation, or livestock.

That is why the calculator asks for minutes per hour rather than assuming the pump runs the whole outage. Multiplying running watts by the full outage length overestimates the energy needed. A pump drawing 1,000 watts that runs 10 minutes of each hour uses about 1000 × 10 / 60 = 167 Wh per hour, not 1,000 Wh.

Estimate a realistic busy case for your household during an outage, and remember that a larger pressure tank means fewer, longer pump cycles. The same on-and-off idea drives a sump pump; the Sump Pump Backup Power Calculator covers that case.

For energy alone, and only if the unit's output and voltage also suit the pump:

• 500 Wh (around 325–400 Wh usable) is not a realistic choice for most well pumps. Units this size rarely have the continuous or surge output a well-pump motor needs, and the energy only covers a short outage with very light water use.
• 1,000 Wh (around 650–800 Wh usable) can cover a few hours for a 120V pump running roughly 10 minutes per hour, but many 1,000 Wh units still fall short on surge output for the motor. Check the ratings.
• 2,000 Wh (around 1,300–1,600 Wh usable) covers a longer outage or heavier water use, and units this size are more likely to have the surge headroom a pump motor needs — for a 120V pump.
• 3,000 Wh and larger is for long outages, heavy household demand, or running the pump alongside other essentials such as those in the Home Power Outage Calculator.

Put your pump's measured running and starting watts and a realistic minutes-per-hour into the calculator above, then confirm the unit's output ratings and voltage before buying.

A capacity class is an energy answer. It tells you a power station probably stores enough watt-hours for your outage. It does not tell you the unit can start and run the pump. Three things can still stop it:

• Continuous output. If the inverter's rated continuous watts are below the pump's running watts, it will overload and shut down, even with a full 3,000 Wh battery.
• Surge output. If the surge / peak rating is below the pump's starting watts, the pump will not start at all.
• Voltage. A 240V pump needs a 240V output. A 3,000 Wh unit with a 120V-only output cannot run it, and no amount of extra capacity changes that.

Treat the recommended capacity class as the first filter, then read the unit's continuous output, surge rating, AC voltage, and outlet type against your pump's numbers. Only all four together confirm compatibility.

Check every item against your pump and your situation before buying:

• Battery capacity (Wh) at least the recommended figure from the calculator above, with margin for a longer or busier outage.
• AC voltage matches your pump — 120V, or a genuine 240V output for a 240V pump. This is the first thing to rule a unit in or out.
• Continuous AC output (W) at or above your pump's running watts, with headroom.
• Startup / surge output (W) at or above your pump's starting watts. This is the rating units most often fall short on for a motor.
• Outlet / output configuration an outlet or connection your pump's wiring can actually use, and a pure sine wave inverter for a motor.
• Recharge time how quickly it refills from a wall outlet or vehicle, so it is ready for the next outage.
• Solar recharge capability if outages in your area can last more than a day — see the Solar Charge Time Calculator and Solar Panel Size Calculator.

A power station in the right capacity class is not confirmation that it can start and run your well pump. Only the continuous output, surge rating, and AC voltage, checked against your pump's own numbers, tell you that.