Best battery backup for a sump pump

The short answer

The best battery backup for a sump pump is a dedicated 12 V backup pump with its own float switch, charger and alarm, beside your primary pump. The Basement Watchdog Big Dog CONNECT moves the most water at 10 feet. The Zoeller Aquanot Spin 508 rates its flow on a real 12.7 V battery. Pair either with an AGM battery.

A primary sump pump runs on household current, so it stops the moment the grid does. A backup system sits in the same pit, slightly higher than the primary, and switches on when the water rises past its own float. Most systems also back up a failed primary pump, a tripped breaker or a stuck switch, which is why many owners treat them as basement insurance rather than outage gear.

At a glance

The picks

Our top two have their own write-ups: read the full Big Dog CONNECT review and read the full Aquanot Spin 508 review.

No. 1Sump pump backup

Basement Watchdog Big Dog CONNECT Battery Backup Sump Pump System (BWD12-120C)

Best for: Highest backup flow and the fullest set of alarms for a busy sump pit

• 2,200 GPH at 10 ft of lift; 3,500 GPH at 0 ft; 16 ft max head
• 12 V DC pump; 20 A multistage smart charger
• Maker runtime: 40 hrs on one BW-27AGM battery, 80 hrs on two (10% duty cycle)
• Alerts for pump activation, battery problems, backup pump failure, loose cable and power or breaker issues
• Tests and exercises the pump weekly; shows battery charge percentage
• USB port for the optional BW-WiFi2 module (text, email or push alerts)
• Dual float switch included; 1.25 in discharge with 1.25 x 1.5 in adaptor
• 2-year limited warranty, up to 4 years with a Watchdog AGM battery and registration

Tradeoffs

• · Battery and Wi-Fi module are sold separately
• · Draws about 20 A at 10 ft of head, so a single battery empties faster in a heavy storm than the runtime figure suggests

Check price on AmazonRead the full reviewSpecs: manufacturer page, checked Nov 1, 2026

No. 2Sump pump backup

Zoeller Aquanot Spin 508 Battery Backup System (508-0005)

Best for: Flow ratings measured on a real battery, plus strong controller diagnostics

• 34 GPM (about 2,040 GPH) at 10 ft at 12.7 volts, per Zoeller's sales sheet; 30 GPM at 10 ft at 12.1 volts and 22 ft shut-off head on its data sheet
• Maker runtime: 2 days of protection at 10% duty cycle; 5.5 hrs continuous pump run
• Alerts for pump activation, pump, switch, battery and charger faults, stuck switch or blocked pipe, air lock and power outage
• Tests the pump on an adjustable schedule; multi-stage charging
• Battery case fits group 27, 29 and 31 batteries; minimum battery is a size 27 deep-cycle with 175 minutes reserve capacity
• 3-year warranty; ProPak versions add an M53 or M98 primary pump

Tradeoffs

• · Deep-cycle battery sold separately; the high-water sensor is an add-on (10-4830)
• · Wi-Fi alerts need the Aquanot Fit 508 version instead

Check price on AmazonRead the full reviewSpecs: manufacturer page, checked Nov 1, 2026

No. 3Sump pump combo

Basement Watchdog Big Combo CONNECT Primary and Battery Backup Sump Pump (CITS-50)

Best for: Replacing an old primary pump and adding a backup in one preassembled unit

• Primary: 1/2 HP, 3,540 GPH at 10 ft (4,400 GPH at 0 ft)
• Backup: 1,850 GPH at 10 ft (2,600 GPH at 0 ft)
• Maker runtime: 48 hrs on one BW-27AGM, 96 hrs on two (10% duty cycle)
• Preassembled primary and battery backup system
• USB port for the optional BW-WiFi2 alert module
• 12 V DC, 1.6 A automatic multistage backup charger

Tradeoffs

• · Basement Watchdog says it does not alert if the backup pump fails and does not run an automatic pump check
• · Battery and Wi-Fi module sold separately; replaces your existing primary pump

Check price on AmazonRead the full reviewSpecs: manufacturer page, checked Nov 1, 2026

No. 4Sump pump combo

Wayne WSS30V Combination Primary and Battery Backup Sump Pump System

Best for: Pre-plumbed primary plus backup with the longest warranty here

• Primary: 1/2 HP cast iron, 3,840 GPH at 10 ft (5,100 GPH at 0 ft)
• Backup: 12 V ESP25, 1,680 GPH at 10 ft; fits sump basins 15 in or wider
• Maker runtime: up to 26 continuous hrs of backup
• Pre-assembled and pre-plumbed with two check valves
• Audible alarm when the backup runs; LED battery status
• 5-year limited warranty

Tradeoffs

• · 12 V battery sold separately (Wayne recommends the 75 Ah WSB1275)
• · No Wi-Fi alerts listed by Wayne

Check price on AmazonRead the full reviewSpecs: manufacturer page, checked Nov 1, 2026

No. 5Sump pump backup

Wayne ESP25 12 Volt Battery Back Up System

Best for: A straightforward add-on backup beside an existing primary pump

• 1,680 GPH at 10 ft of discharge lift; 2,900 GPH at 0 ft; 600 GPH at 15 ft
• Maker runtime: up to 26 continuous hrs with a 75 Ah battery at 5 gallons per minute of inflow
• Audible alarm when the 12 V pump runs; LED battery charge indicator
• Smart charger included; recommended battery is the Wayne WSB1275 (75 Ah, sold separately)
• Fits 1-1/2 or 1-1/4 in discharge; 2-year limited warranty

Tradeoffs

• · Fewer fault alerts than the Basement Watchdog CONNECT and Zoeller controllers
• · Wayne also sells an ESP25n with different specs, so match the model on the listing

Check price on AmazonRead the full reviewSpecs: manufacturer page, checked Nov 1, 2026

No. 6Sump pump backup

Basement Watchdog Emergency Battery Backup Sump Pump System (BWE)

Best for: A basic, low-cost backup for a pit that rarely runs hard

• 1,000 GPH at 10 ft of lift; 2,000 GPH at 0 ft; 11 ft max head; draws 9.5 A at 10 ft
• Maker runtime: up to 80 hrs on one BW-27AGM battery (10% duty cycle)
• Alerts for pump activation, battery replacement, discharged battery, loose cable and power or breaker issues
• Controller, charger, dual float switch and battery box included
• 1.25 in discharge with 1.25 x 1.5 in adaptor

Tradeoffs

• · Half the 10 ft flow of the Big Dog CONNECT
• · Basement Watchdog says it does not alert if the backup pump itself fails
• · Small 0.4 A charger refills a drained battery slowly; battery sold separately

Check price on AmazonRead the full reviewSpecs: manufacturer page, checked Nov 1, 2026

What to look for in a battery backup sump pump

Flow at 10 feet, not at 0 feet

Box fronts quote gallons per hour (GPH) at 0 feet of lift. Your water has to climb from the pit to the discharge, usually several feet, so compare the 10-foot figure. The Big Dog CONNECT drops from 3,500 GPH at 0 feet to 2,200 GPH at 10 feet. Zoeller notes that some makers rate DC pumps on a bench power supply, which it says can show 40% to 80% more flow than a real battery delivers. Zoeller rates its 508 on a 12.7-volt battery.

Battery type: AGM, wet cell or lithium

• AGM (absorbed glass mat): sealed and maintenance-free. Basement Watchdog recommends its BW-27AGM for all three of its systems here.
• Wet (flooded) deep-cycle: usually cheaper per amp-hour but needs its water level checked. Basement Watchdog says the Big Dog CONNECT works with wet-cell batteries, and Zoeller sells both an AGM and a wet battery for the 508.
• Lithium: none of the controllers above list a lithium battery as supported. A charger built for lead-acid can mistreat a lithium pack, so stay with the battery type the maker specifies.
• Size: Zoeller's minimum is a size 27 deep-cycle battery with 175 minutes of reserve capacity. Its battery case fits group 27, 29 and 31. Two batteries in parallel roughly double runtime on systems that allow it (Big Dog CONNECT and Big Combo CONNECT do, via a jumper cable).

Runtime claims need a duty cycle

A pump does not run nonstop. Basement Watchdog and Zoeller quote runtime at a 10% duty cycle, meaning the pump runs about 6 minutes in every hour. Wayne quotes its 26 hours at 5 gallons per minute of inflow. In a storm that keeps your pit filling fast, the pump runs far more often and the battery empties sooner. Zoeller's sheet gives the honest worst case: 5.5 hours of continuous pumping.

Alarms you will actually hear

The controller is what separates a good system from a pump on a battery. Look for alerts when the backup runs, when the battery is weak or disconnected, and when the backup pump itself fails. Basement Watchdog states that the BWE and the Big Combo CONNECT do not alert on backup pump failure; the Big Dog CONNECT does. If no one is home during storms, a Wi-Fi alert module (or Zoeller's Fit version with built-in Wi-Fi) turns a basement alarm into a phone notification.

Running a primary pump from a power station or generator

A second route is to keep your primary pump running on backup power. This covers outages only, not a failed primary pump, but it moves more water than any 12 V backup. The math starts with the pump's nameplate.

1. 01Find the amps on the pump label or spec sheet. Zoeller lists its M53 (3/10 HP) at up to 9.7 amps on 115 V.
2. 02Convert to watts: 9.7 A x 115 V = about 1,116 W running.
3. 03Add startup surge. Induction motors draw a brief inrush above their running current when they start. Choose a power station or generator whose surge rating sits well above the running figure, and check the pump maker's starting or locked-rotor amps if it publishes them.
4. 04Estimate runtime. A 1,024 Wh station with about 85% usable gives roughly 870 Wh. At 1,116 W, that is about 47 minutes of actual pumping. At a 10% duty cycle, that stretches to about 7.8 hours of storm.

Stations with 1,800 W continuous output, such as the Anker SOLIX C1000 or EcoFlow DELTA 3 Plus, sit in the right range for a 1/3 HP class pump. Read our power station for a refrigerator guide for how we size these units, or use the BackupPowerAI sizing quiz. For multi-day storms, a generator refuels faster than a station recharges; see best generator for a power outage.

Installation and maintenance

• Plumbing: the backup discharge usually tees into the existing line, with its own check valve so water does not cycle back into the pit. Wayne's WSS30V ships pre-plumbed with two check valves. If you are not comfortable cutting PVC, hire a plumber.
• Electrical: the charger needs an outlet near the pit. If you need a new one, have an electrician add it to local code.
• Pit size: Zoeller recommends a pit at least 18 inches across and 22 inches deep for the 508. Two pumps and two floats need room to move.
• Testing: Big Dog CONNECT and Zoeller controllers exercise the pump on a schedule. Still, pour a bucket of water into the pit every few months with the primary unplugged and confirm the backup starts and the alarm sounds.
• Battery replacement: replace the battery when the controller flags it, and note the install date on the case.

Honest tradeoffs

A 12 V backup pump moves less water than a 1/2 HP primary. The Wayne WSS30V primary is rated at 3,840 GPH at 10 feet; its backup at 1,680. If your pit fills faster than the backup can empty it, you need a bigger backup, a second battery, or primary power from a station or generator. A combo unit simplifies installation but means replacing a primary pump that may still work. And no backup system helps if the discharge line is frozen or blocked, which is why Zoeller's controller watches for continuous runs.

For the rest of the outage plan, see how long power outages last and our hurricane power outage prep guide.