Battery backup for a sump pump: what actually works, sized from the pump's own numbers

A sump pump is the appliance people buy backup power for with water already in the pit, so the verdict comes first and the arithmetic under it. The short version: any of the five picks runs a residential sump pump; how long it runs is the only question, and the runtime table answers it. The rest of this page is what the box will not tell you.

The pump’s real numbers, not the horsepower

Horsepower is a poor guide to draw. Zoeller’s 3/10 HP M53, the most common basement pump in the country, is rated 9.7 A at 115 V, about 1,116 W running. Zoeller’s 1/2 HP M98 is rated 9.4 A, slightly less. Liberty’s 1/3 HP 257 is rated 5.2 A, under 600 W. Three “1/3 to 1/2 HP” pumps span almost a factor of two, which is why the table above is built on one named pump and the sump pump wattage guide lists the others. Read the amps on your own pump’s label and multiply by 115.

Surge is the margin, not the emergency

Every pump article warns that motors draw three to five times their running current at start. That is true of capacitor-start motors, and it is why the surge rating decides whether a battery works at all for a well pump. Submersible sump pumps are gentler: Liberty publishes 8 A locked rotor against 5.2 A running, a ratio of about 1.5. Zoeller does not publish one. So the honest rule for a sump pump is that the inverter’s continuous rating should clear the running draw with real margin, and the surge rating should be large enough to cover a pump you have not measured. Every pick here has at least 1,800 W continuous and 3,000 W surge against an 1,116 W pump. The mechanics are in running watts versus surge watts.

Sizing the night

At a 25% duty cycle the M53 averages 279 W, so eight hours of steady rain is about 2,230 Wh delivered. Inverters lose roughly 15% turning battery watt-hours into outlet watt-hours, which is why the spec strips print a usable figure beside the sticker and why the table’s columns are usable watt-hours; the reasoning is in usable capacity versus advertised. A 2 kWh unit covers most of a night, and the 2 kWh roundup ranks that whole class. If your pump runs more than a quarter of the time, or the storm is longer than a night, the expandable picks are the answer: the DELTA 2 Max and the DELTA 3 Plus both accept extra batteries.

What the certification chips are telling you

Every card on this page says “none stated by the maker” because none of the five makers prints a UL 2743 listing on its product page as of the check date. Several cite a UL94 flame rating for the casing, which is a materials test, not a product safety listing, and we do not upgrade one to the other. If a listing matters to you, and for a device that lives in a wet basement it should, look for the UL mark on the unit itself and ask the maker in writing.

What we left out

Jackery’s Explorer 2000 Plus and 2000 v2 both fit this class on paper and both showed as currently unavailable at Amazon on the check date, so neither is ranked; they return to consideration when they return to stock. The picks are all lithium iron phosphate, which is what lets a unit sit charged for months between storms; the LiFePO4 versus NMC guide explains the trade. Dedicated DC backup pumps are a different product and will get their own page. Wiring a power station into a circuit is transfer-switch territory: read the callout at the top, and hand that part to a licensed electrician.