What Size Battery Bank Do I Need for a Sump Pump Backup?
A sump pump bank is sized per hour of actual run time. The inverter is sized from the starting watts.
Direct answer: Honda lists a 1/2 hp sump pump at 1,050 W running and 2,150 W starting, which is 87.5 Ah per hour of run time at 12 V (1,050 ÷ 12). A 1/3 hp pump is 800 W running and 1,300 W starting, or 66.7 Ah per hour (Honda wattage guide). The inverter continuous rating has to clear the running watts and the surge rating has to clear the start; a 2,000 W inverter then needs at least 200 Ah by the per-1,000-watt battery rule (Battle Born inverter guide).
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Champion's chart in the source notes gives a 1/2 hp sump range only. The 1/3 hp row is Honda's.
How to size it
Count hours the pump actually runs, not hours the basement is occupied. Amp-hours per hour at 12 V are running watts ÷ 12 (Battle Born battery-bank guide uses watt-hours ÷ voltage; one hour makes watt-hours equal the watts).
Honda 1/2 hp: 1,050 ÷ 12 = 87.5 Ah for each hour of running. Honda 1/3 hp: 800 ÷ 12 = 66.7 Ah per hour (Honda wattage guide).
Champion's 1/2 hp running range is 800–1,050 W, so the per-hour bank spans the same 66.7–87.5 Ah. Starting watts are 1,300–2,150 W, matching Honda's 2,150 W at the top (Champion wattage chart).
A second hour doubles the energy. Two hours of the 1/2 hp Honda pump are 175 Ah at 12 V. Two days of storage, if the pump ran the same amount each day, would double the day (Battle Born battery-bank guide).
Inverter continuous rating must be at least the running watts, and the surge rating at least the start (Battle Born inverter guide). For the 1/2 hp Honda pump that is at least 1,050 W continuous and at least 2,150 W surge. A 1,000 W continuous inverter is short of 1,050 W running. A 2,000 W continuous inverter is above 1,050 W, and the battery rule for that inverter is at least 200 Ah (Battle Born inverter guide).
Scaled to the running load itself, 1,050 ÷ 1,000 × 100 = 105 Ah. That is the rule applied to 1,050 W. It is not a substitute for the 200 Ah figure if the inverter you install is 2,000 W (Battle Born inverter guide).
Current tells you whether one battery can carry the pump. Running current is 87.5 A for the 1,050 W pump, under the 100 A continuous rating. Starting current is 2,150 ÷ 12 = 179.2 A, under the 200 A rating allowed for 30 seconds (Battle Born 100 Ah manual). The 1/3 hp start is 1,300 ÷ 12 = 108.3 A, also under that 30-second rating. So the pump current can fit one battery even when a 2,000 W inverter's amp-hour rule asks for 200 Ah. Those are different limits: energy and inverter capacity on one side, pump amps on the other.
Many 12 V systems running larger inverters are recommended to use at least two batteries (Battle Born inverter guide). That sentence attaches to the larger inverter, not to the 1/3 hp pump's 66.7 A running draw.
Lead-acid at about 50% depth of discharge doubles the per-hour nameplate. One hour at 87.5 Ah usable is about 175 Ah of lead-acid (Battle Born chemistry guide). Lithium in the manual is 100% usable (Battle Born 100 Ah manual).
A generator, solar, alternator, or shore power can refill the bank and reduce how many hours you must store (Battle Born battery-bank guide). Generator wattage for the same pump is the full breakdown linked above.
If the pump shares a bank with other loads, add those watt-hours on the battery bank calculator.
A 2,000 W inverter needs at least 200 Ah (Battle Born inverter guide). Separately, 1,050 ÷ 1,000 × 100 = 105 Ah if you scale the rule to the running watts alone. The per-hour energy, 87.5 Ah, is a third number and grows with hours of pumping.
Will one battery carry the pump current?
The 1,050 W running load is 87.5 A, under 100 A continuous, and the 2,150 W start is 179.2 A, under a 200 A 30-second surge (Battle Born 100 Ah manual). A larger inverter can still push the amp-hour rule to 200 Ah.