Sump pump battery backup
A battery backup sump pump is a second, 12-volt pump that sits in the pit next to your main pump and runs from a deep-cycle battery when the power goes out or the main pump fails.
The 11 systems we compared move 1,000–2,300 gallons per hour at a 10-foot lift, and the two makers that publish continuous runtime (Liberty and Zoeller) list 5.5 hours non-stop on one fully charged battery — which stretches to days, because the pump only runs when water comes in.[3][1] Two batteries roughly double that.
Use the estimator below to see how long one would last in your pit.
Battery backup systems compared
All figures come from the manufacturers' own documents. Brands test differently, so read the "test condition" column before comparing flow numbers.
Backup sump pump comparison
| System | Flow at 10 ft (GPH) | Test condition | Runtime (manufacturer) | Batteries | Charger | Wi-Fi alerts | Source |
|---|---|---|---|---|---|---|---|
| Basement Watchdog Big Combo CONNECT CITS-50 | 1,850 | DC backup pump; test condition not stated | not published 48 hours at 10% duty cycle with one BW-27AGM battery; 96 hours with two (manufacturer, backup pump only) | up to 2, sold separately | 1.6 A | optional — BW-WiFi2 module sold separately via USB data port; no fees | [9] |
| Basement Watchdog Big Dog CONNECT BWD12-120C | 2,200 | test condition not stated | not published 40 hours at a 10% duty cycle with one BW-27AGM battery; 80 hours with two (manufacturer product page; runtimes vary with water flow) | up to 2, sold separately | 20 A | optional — BW-WiFi module sold separately; no fees | [5][6] |
| Basement Watchdog Emergency BWE | 1,000 | test condition not stated | not published about 80 hours at 10% duty cycle with one BW-27AGM battery (manufacturer) | up to 1, sold separately | 0.4 A | none | [10] |
| Basement Watchdog Special CONNECT BWSP | 1,850 | test condition not stated | not published 48 hours at 10% duty cycle with one BW-27AGM battery; 96 hours with two (manufacturer) | up to 2, sold separately | 1.6 A | optional — BW-WiFi2 module sold separately via USB data port; no fees | [11] |
| Liberty Pumps StormCell 442-10A 442-10A, 442-10A-EYE | 1,800 | fixed 12.1 V at battery terminals (30 GPM) | 5.5 h continuous — continuous run at 10 ft lift, one StormCell Group 31 wet-cell battery (~8,000 gallons) 8 days at 10 ft lift, pumping 10 gallons per cycle, 4 cycles an hour (manufacturer brochure LLIT006746-R07/23) | up to 1 (group 27/31), sold separately | 10 A | optional — -EYE models add NightEye app alerts, no subscription | [3] |
| Liberty Pumps StormCell 442-25A 442-25A, 442-25A-EYE | 1,800 | fixed 12.1 V at battery terminals (30 GPM) | 11 h continuous — continuous run at 10 ft lift, two StormCell Group 31 wet-cell batteries (~16,000 gallons) 16 days at 10 ft lift, pumping 10 gallons per cycle, 4 cycles an hour (manufacturer brochure LLIT006746-R07/23) | up to 2 (group 27/31), sold separately | 25 A | optional — -EYE models add NightEye app alerts, no subscription | [3] |
| Wayne ESP25-N ESP25-N | 1,680 | test condition not stated | not published | up to 1, sold separately | — | none | [4][7] |
| Wayne WSM3300 WSM3300 | 2,300 | test condition not stated | not published up to 26 hours of protection at 5 gal/min incoming water with a 75 Ah battery (manufacturer) | up to 1, sold separately | — | none — Auto-dialer calls up to 3 programmed phone numbers on standard phone line (phone service required) | [12][13] |
| Wayne WSS30V WSS30V | 1,680 | DC backup pump (ESP25); test condition not stated | not published up to 26 continuous hours of protection (manufacturer; test condition not fully stated) | up to 1, sold separately | — | none | [14] |
| Zoeller Aquanot Fit 508 508-0014, 508-0015, 508-0016, 508-0020 | 1,800 | 12 V deep-cycle battery, 175-minute reserve capacity (30 GPM; 2018 data sheet) | 5.5 h continuous — fully charged battery with 175-minute reserve capacity, pump running continuously 2 days of battery protection at a 10% duty cycle (manufacturer) | up to 2 (group 27/29/31), sold separately | 7 A | built-in — Z Control app; self-test schedule is adjustable | [1][2] |
| Zoeller Aquanot Spin 508 508-0005, 508-0006, 508-0007 | 2,040 | at 12.7 V (34 GPM; 2024 sales sheet, FM3525) | 5.5 h continuous — fully charged battery, pump running continuously 2 days of battery protection at 10% duty cycle (manufacturer) | up to 2 (group 27/29/31), sold separately | — | none — Wi-Fi is available on Fit 508 only; Spin has no built-in or optional Wi-Fi module listed | [2][15] |
Why the flow numbers don't line up
Zoeller states that its chart is measured on a real 12-volt deep-cycle battery and warns that some manufacturers publish DC pump test data that "can reflect a performance of 40% – 80% higher than actual battery powered performance."[1] Liberty publishes two curves — one at a fixed 12.1 V and one with AC power on — and the difference at 10 ft is 30 vs 35 gallons per minute.[3]
Retail listings add to the confusion: one store lists the Wayne ESP25 at 3,300 GPH,[7] while Wayne's own ESP25-N manual shows 2,900 GPH at 0 ft and 1,680 GPH at 10 ft.[4] Basement Watchdog's brochure gives 2,200 GPH at 10 ft without stating the test condition.[5]
What to do with this: compare at the same lift (10 ft is the common reference), prefer figures measured on battery power, and treat "0 ft" maximums as marketing numbers — your pump never lifts water zero feet.
How long will a sump pump battery last?
The battery only drains while the pump runs, so protection time depends on how much water enters your pit. This estimator scales each manufacturer's continuous-runtime figure by how often your pump actually runs.
Don't know gallons per cycle? Work it out from your pit
How it's calculated: pump running time per hour = (gallons per cycle × cycles per hour) ÷ the system's flow at 10 ft on battery. Days = manufacturer's continuous runtime ÷ that share ÷ 24. Treat the result as an upper bound: it ignores standby drain, battery age and cold. As a check, Liberty's own figure for 10 gallons × 4 cycles an hour at 10 ft is 8 days for the 442-10A,[3] while this formula gives about 10 — real-world numbers come in lower. Wayne and Basement Watchdog don't publish a continuous-runtime figure in the documents we found, so they aren't in the estimator yet.
What to look for
- Flow at your lift, on battery. Measure from the pump to where the pipe exits the house; use the 10 ft figure if you're unsure.
- Battery capacity. Systems take group 27 or 31 deep-cycle batteries (Zoeller also lists group 29); bigger reserve capacity means longer runtime.[1][3] Liberty's 442-25A, Zoeller's Aquanot and Basement Watchdog's Big Dog can run a second battery.
- Alerts. A backup that runs silently in an empty house doesn't help much. Zoeller's Fit models have Wi-Fi built in; Liberty (-EYE models) and Basement Watchdog offer it as an option.[2][3][5]
- Self-testing. Basement Watchdog tests the pump weekly; Zoeller's Aquanot lets you set the schedule.[5][2]
- Pit size. Zoeller recommends a pit at least 18 in. wide and 22 in. deep for its backup system.[1]
Alternatives to a battery backup pump
Generator or power station. If you'd rather power your existing pump, a 1/3 HP sump pump needs about 800 running watts plus 1,300 starting watts, and a 1/2 HP pump about 1,050 plus 2,200.[8] That's typical-value math — check your pump's nameplate — and the surge is what trips up small inverters. Size it with the generator size calculator.
Water-powered backup. Uses city water pressure instead of a battery, so runtime isn't limited by a battery — but it needs municipal water and may require a backflow preventer. See water-powered sump pump backups for requirements and models.
More in this guide
Frequently asked questions
How long does a sump pump battery backup last?
Liberty and Zoeller both publish a continuous runtime of 5.5 hours on a fully charged group 27 or group 29 deep-cycle battery.[3][1] Because the pump only runs when water rises in the pit, real-world protection typically stretches to several days during a storm. Use the estimator above to see how long for your pit.
Do I need a battery backup if my main sump pump is new?
Yes. A main sump pump — new or old — stops working the moment grid power fails. A battery backup sump pump runs on a 12-volt deep-cycle battery and keeps pumping through outages regardless of your main pump's age. Installation involves adding a second pump head and discharge line in the same pit.
What is the difference between a battery backup and a water-powered backup sump pump?
A battery backup runs from a 12-volt deep-cycle battery. A water-powered backup uses city water pressure to eject water and has no battery to replace. Water-powered systems require a pressurized municipal supply and a backflow preventer.[1] See the water-powered backup guide for pressure requirements and models.
Can I add a battery backup sump pump to my existing pump?
Yes. Models like the Liberty 442, Zoeller Aquanot, and Basement Watchdog Big Dog are designed to sit beside a primary pump in the same pit on a separate discharge line.[3][1][5] The backup activates only when the main pump cannot keep up or when power fails.
How often should I test a sump pump battery backup?
Basement Watchdog performs an automatic self-test weekly.[5] Zoeller Aquanot lets you schedule the test interval.[2] For systems without automatic testing, pour water into the pit quarterly to confirm the float switch activates and the pump runs.
What battery group size do sump pump backups use?
Most backup systems take a group 27 or group 31 deep-cycle battery; Zoeller also lists group 29.[1][3] Larger group sizes hold more reserve capacity and extend runtime. Batteries are sold separately from most systems. See sump pump battery types and sizing for a full breakdown.
Sources
- Model 508 12-Volt Backup technical data sheet (FM3124) — Zoeller Co., document dated 2018-09. Accessed 2026-11-03. Manufacturer data sheet hosted by a retailer (SupplyHouse).
- Battery Backup Systems sales sheet (FM3525) — Zoeller Co., document dated 2024-12. Accessed 2026-11-03.
- 442-Series Battery Back-Up Pumps brochure (LLIT006746-R07/23) — Liberty Pumps, document dated 2023-07. Accessed 2026-11-03. Manufacturer brochure hosted by a retailer.
- ESP25-N Operating Instructions and Parts Manual (600001-001 B) — WAYNE / Scott Fetzer Company, document dated 2018-08. Accessed 2026-11-03. Manufacturer manual hosted by Home Depot.
- Big Dog CONNECT battery backup sump pump brochure (BWD12-120C) — Basement Watchdog, document dated 2018-06. Accessed 2026-11-03. Manufacturer brochure hosted by Home Depot.
- Big Dog CONNECT Battery Backup Sump Pump product page (BWD12-120C) — Basement Watchdog. Accessed 2026-11-03. Current product page. Lists max head 16 ft, 2,200 GPH at 10 ft, 20 A charger, 40-hour runtime at 10% duty cycle with one BW-27AGM battery.
- Wayne ESP25 product listing — PlumbersStock. Accessed 2026-11-03.
- Generator Sizing Considerations — Eugene Water & Electric Board (EWEB). Accessed 2026-11-03.
- Big Combo CONNECT Primary and Battery Backup Sump Pump product page (CITS-50) — Basement Watchdog. Accessed 2026-11-03.
- Emergency Battery Backup Sump Pump System product page (BWE) — Basement Watchdog. Accessed 2026-11-03.
- Special CONNECT Battery Backup Sump Pump System product page (BWSP) — Basement Watchdog. Accessed 2026-11-03.
- WSM3300 12-Volt Battery Back-Up Sump System product page — WAYNE / Scott Fetzer Company. Accessed 2026-11-03.
- WSM3300 Battery Back-Up Systems specification sheet — WAYNE / Scott Fetzer Company, document dated 2016-05. Accessed 2026-11-03. Manufacturer spec sheet. Shows 14-day self-test interval and performance curve.
- WSS30V 1/2 HP Sump Pump with Battery Backup product page — WAYNE / Scott Fetzer Company. Accessed 2026-11-03.
- Aquanot Spin 508 product page — Zoeller Co.. Accessed 2026-11-03.