How many watts does a sump pump use? Four real pumps, from the nameplate

The number on the box is horsepower, and horsepower is not watts. Two pumps sold as “1/3 HP” can differ by almost a factor of two in draw, because what matters is the motor design and the amps printed on the nameplate. This page reads four common pumps from their makers’ own spec sheets so you can size backup power from a real figure. The sizing itself, with a runtime table and ranked units, is on the battery backup for a sump pump page.

Four pumps, from the makers’ sheets

Watts here are nameplate amps multiplied by nameplate volts. That is volt-amps, strictly, but it is the figure an inverter has to supply and the one every backup calculation should start from.

Pump	Rated HP	Nameplate	Running watts	Motor
Zoeller M53	3/10	9.7 A at 115 V	1,116 W	Shaded pole
Zoeller M98	1/2	9.4 A at 115 V	1,081 W	Not stated on the page
Liberty 257	1/3	5.2 A at 115 V	598 W	8 A locked rotor published
Wayne CDU980E	3/4	10 A at 120 V	1,200 W	Not stated on the page

Spec sheets checked September 9, 2026. Two things stand out. Zoeller’s 1/2 HP pump draws slightly less than its 3/10 HP pump, so the horsepower label tells you about the pump’s lifting ability, not its appetite. And Liberty’s 1/3 HP pump draws about half of Zoeller’s, which is the difference between a night on a 2 kWh battery and a night and a half.

Starting draw: the one maker that says

Every article about backup power warns that motors pull three to five times their running current for a fraction of a second at start-up. That is true of capacitor-start induction motors, which is why it matters so much for well pumps and compressors; Franklin’s published figures are in the well pump wattage guide. Submersible sump pumps are mostly shaded-pole or permanent-split-capacitor designs, which start more gently, and Liberty is the one maker in this table that publishes the number: 8 A locked rotor against 5.2 A running, a ratio of about 1.5. Zoeller and Wayne do not publish one, so for those pumps the honest plan is margin: an inverter whose continuous rating clears the running watts comfortably and whose surge rating is large enough to cover a start you have not measured.

From watts to a night

Watts are a rate. A battery is bought in watt-hours, so the question is how much of each hour the pump actually runs. That duty cycle is the assumption every sizing depends on, and it is the one you can measure: time the pump over ten minutes during a rain.

Duty cycle	What it looks like	Average draw (M53)	Eight hours
10%	Steady drizzle, six minutes an hour	112 W	893 Wh
25%	Steady rain, fifteen minutes an hour	279 W	2,232 Wh
50%	Heavy rain, half of every hour	558 W	4,464 Wh
100%	Flash flood, never stops	1,116 W	8,928 Wh

Those are watt-hours delivered at the outlet. A battery has to hold more than that, because the inverter loses roughly 15% converting battery watt-hours into outlet watt-hours; the usable capacity guide walks through that step. Divide the delivered figure by 0.85 to get the usable battery you need: a 25% night on the M53 wants about 2,626 Wh of usable battery, which is why the 2 kWh class is the honest floor for a night and the 1 kWh class is a bridge.

The same method works for any appliance with a nameplate; the refrigerator wattage guide uses the EnergyGuide label instead, because a fridge’s draw is an average of a cycling compressor, and the CPAP page works from the maker’s published typical and peak figures.

Reading your own label

Find the plate on the pump body or the tag on its cord. It lists volts and amps; multiply them. If it lists locked-rotor amps as well, multiply those by volts too and you have the surge figure most makers leave out. If the plate is unreadable, the model number on the same plate finds the maker’s spec sheet, which is where every number above came from.