Sump pump sizing Last reviewed June 2026
Sump Pump Battery Backup: What Size Do You Need?
By the WhatSize editorial team 4 min read Standards-reviewed
A sump pump battery backup system should match your primary pump's HP and head pressure. For a 1/2 HP primary pump, you need a battery backup rated for 2,000-3,000 GPH with a deep-cycle marine battery.
Why Every Finished Basement Needs Backup
Sump pump failures happen during the worst possible time — when it's raining hardest and the power is most likely to go out. A battery backup sump pump sits in the same pit as your primary pump and kicks in automatically when the power fails or the primary pump fails.
For sizing, the backup pump doesn't need to match the primary pump's capacity. A battery backup rated at 2,000-3,000 GPH is sufficient for most residential applications — enough to keep water from rising while you wait for power to return. The key specification is runtime: look for systems that provide at least 4-6 hours of continuous operation on a fully charged battery.
Top picks for this size
These Amazon-available picks meet the calculated spec. For region-specific or licensed contractor-installed options, consult a local professional. See our Disclaimers.
Battery Backup Sizing Recommendations
- 1/3 HP primary pump: Wayne WSS30V battery backup (~$259) — 2,000 GPH, includes battery
- 1/2 HP primary pump: Wayne WSS30V + group 27 deep cycle battery (~$300-350) — 2,000 GPH, longer runtime
- 3/4 HP primary pump: Zoeller Aquanot 508 (~$450) — 2,400 GPH, heavy-duty design
- Multiple backup strategy: Battery backup + water-powered backup (uses municipal water pressure) for unlimited runtime during extended outages
Frequently asked questions
What size battery backup for a sump pump?
On a fully charged group 27 deep-cycle marine battery, most backup pumps run 4-8 hours continuously. If the pump cycles on and off (normal operation), the battery can last 12-24+ hours. Consider a dual-battery setup for extended outages.
How often do I need to replace the backup battery?
Deep-cycle marine batteries last 3-5 years. Replace proactively at the 3-year mark — a battery that fails during a storm is worse than no battery. Keep a log of the installation date.
Can I install a battery backup sump pump myself?
Most battery backup systems are designed for DIY installation. The backup pump mounts on a bracket in the sump pit alongside the primary pump, and the battery connects with included cables. Plan for 1-2 hours of installation time.
You might also need
More sump pump sizing guides
- What Size Sump Pump for a Basement? Complete Guide
- 1/3 HP vs 1/2 HP Sump Pump: Which Size Do You Need?
- Sump Pump Sizing for Heavy Rain Areas
- Sump Pump Installation Guide: Pipe Size, Check Valves & More
- Sump Pump GPH Chart: Capacity by Horsepower & Lift
- Sump Pump Battery Backup Sizing + Runtime Chart