Sump Pump Backup Power Calculator

The energy side of this uses the same method as the main Power Station Size Calculator. The difference for a sump pump is that two more specifications decide whether a unit can back it up, and it has to pass all three:

• Battery capacity, in watt-hours (Wh), is how much energy is stored. 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. It has two parts: a continuous rating for the running load, and a higher surge / peak rating for the brief spike when the motor starts.

A large battery with a small inverter will not run the pump. A powerful inverter with a small battery will start the pump but drain quickly. The calculator above reports all three numbers separately so you can match a power station to each one. To see how long a given unit would last against a steady load, use the Power Station Runtime Calculator.

Running watts is the steady power a sump pump draws while its motor is turning. This is the figure that 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 lasting a fraction of a second — as the motor overcomes inertia and begins to spin. It can be considerably higher than the running figure. A power station's inverter has to deliver that spike or the pump will not start, no matter how much battery is behind it.

The gap between the two depends on the motor type, the pump, and how it starts, so it is not something this calculator will estimate for you. Both numbers are usually printed on the pump's rating label or listed in its manual and on the manufacturer's product page — enter what you find there.

Need backup sizing for a household well pump instead? The Well Pump Backup Power Calculator uses the same running-versus-starting approach and adds a 120V versus 240V check.

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

• The rating label on the pump housing or motor. It may list watts directly, or amps and volts — watts is roughly amps multiplied by volts. Look for a separate starting or locked-rotor figure.
• The owner's manual or the specifications section of the manufacturer's product page, which often lists running and starting wattage explicitly.
• A plug-in watt meter on the pump's outlet. Watch the reading while the pump cycles: the steady value is the running figure, and a good meter will also capture a peak. Note that a very brief inrush can be higher than a meter shows.

Horsepower on its own does not give a reliable wattage. Two pumps with the same “1/2 HP” label can draw noticeably different running and starting watts depending on the motor and design, so this calculator asks for the measured or spec-sheet numbers rather than converting from horsepower.

A sump pump does not run continuously. It switches on when the pit fills to the float level, empties it in a minute or two, and switches off again. Over an hour it might run only a few minutes in dry weather, or many minutes during heavy rain or snowmelt.

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, often by a lot. A pump drawing 800 watts that runs 10 minutes of each hour uses about 800 × (10 / 60) = 133 Wh per hour, not 800 Wh.

Estimate the busiest realistic case for your basement — the run time during the kind of storm that also causes the outage — rather than a calm day. The same on-and-off idea applies to a refrigerator; the Refrigerator Power Station Calculator covers duty cycle and startup surge for that case.

It depends on the pump's running watts, how many minutes of each hour it runs, and the usable energy in the power station (its rated capacity minus inverter loss and any reserve). Divide usable watt-hours by the pump's average hourly energy use to get the number of hours of backup.

For a pump drawing 800 watts that runs 10 minutes per hour, average use is about 133 Wh per hour. A 1,000 Wh power station with roughly 650–800 Wh usable would then cover about 5 to 6 hours; a 2,000 Wh unit about 10 to 12 hours. In a heavy storm the pump may run 20 minutes or more per hour, which cuts those times roughly in half.

These are averages. Actual runtime also depends on temperature, battery age, the real inrush at each start, and how often the pump cycles. Enter your own figures in the calculator above, and use the Power Station Runtime Calculator to explore a specific unit against a steady load. If you also want to back up a refrigerator, lights and other essentials in the same outage, the Home Power Outage Calculator adds them all into one estimate.

The honest answer is “it depends on the pump and the outage, and on the inverter, not just the battery.” As rough energy guidance:

• 500 Wh (around 325–400 Wh usable) is only enough for a short outage with light pump activity, and many units this size do not have the continuous or surge output for a typical sump pump motor. Treat it as a stopgap.
• 1,000 Wh (around 650–800 Wh usable) can cover a few hours for a pump running roughly 10 minutes per hour. It is often the smallest size worth considering, and only if its inverter meets the pump's running and starting watts.
• 2,000 Wh (around 1,300–1,600 Wh usable) covers a longer outage or heavier pump cycling, and units this size are more likely to have the surge headroom a motor needs.
• 3,000 Wh and larger is for long outages, storm-level pump activity, or running the pump alongside other essentials.

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

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

• Battery capacity (Wh) at least the recommended figure from the calculator above, with some margin for a longer or busier outage.
• 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 specification units most often fall short on for motor loads.
• AC outlet type a standard grounded outlet that matches your pump's plug, and a pure sine wave inverter for a motor.
• Recharge time how quickly it refills from a wall outlet or a vehicle, so it is ready for the next outage.
• Solar input 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 pump. Only the continuous and surge output ratings, checked against your pump's own numbers, tell you that.