Sump pump sizing Last reviewed June 30, 2026

Sump Pump Battery Backup: What Size Do You Need?

By the WhatSize editorial team 4 min read Standards-reviewed

TL;DR

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.

Looking for a different size? See What Size Sump Pump for a Basement? Complete Guide or browse all sump pump sizing guides.

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

WAYNE CDU800 1/2 HP Submersible Sump Pump – Cast Iron & Stainless Steel, Vertical Float Switch, 5,100 GPH, 3-Year Warranty #ad

★★★★ 4.4 · 1K+ · $159

Amazon

Liberty Pumps 287 1/2-Horse Power 1-1/2-Inch Discharge 280-Series Automatic Submersible Sump Pump with VMF Switch #ad

★★★★☆ 4.5 · 247+ · $312

Amazon

Zoeller M98 Flow-Mate Sump Pump - 1/2 HP - Cast Iron - LED Power Plug - High Flow - Vortex Impeller - Stainless Steel Hardware - Factory Tested and Float Operated - Submersible Sump Pump - 115V #ad

★★★★☆ 4.6 · 1K+ · $304

Amazon

Zoeller Waste-Mate 267 Sewage Pump, 1/2 HP – Heavy-Duty Submersible Sewage, Effluent or Dewatering Pump #ad

★★★★ 4.4 · 130+ · $412

Amazon

Liberty Pumps P382LE51 1/2-Horse Power 24 by 24-Inch Pro380 Series Simplex Sewage System #ad

★★★★☆ 4.5 · 210+ · $700

Amazon

Liberty Pumps 457-2 Vertical Magnetic Float 1/2 HP Submersible Sump Pump, BLACK #ad

★★★★ 4.4 · 57 · $257

Amazon

Liberty Pumps 405 Commercial 1/2 HP Drain Pump, Metal Plastic #ad

★★★★☆ 4.5 · 142+ · $465

Amazon

Zoeller Waste-Mate 267-0001 Sewage Pump, 1/2 HP Automatic – Heavy-Duty Submersible Sewage, Effluent or Dewatering Pump #ad

★★★★ 4.4 · 948+ · $462

Amazon

Liberty Pumps 457-2 450-Series 1/2 hp Submersible Sump Pump #ad

★★★★ 4.4 · · $281

Amazon

Zoeller M98 Flow-Mate Sump Pump - 1/2 HP - Cast Iron - LED Power Plug - High Flow - Vortex Impeller - Stainless Steel Hardware - Factory Tested and Float Operated - Submersible Sump Pump - 115V #ad

★★★★☆ 4.6 · 160+ · $350

Amazon

Superior Pump 91570 3300GPH Thermoplastic Submersible Utility Pump with 10-Foot Cord, 1/2 HP #ad

★★★★ 4.3 · 2K+ · $112

Amazon

WAYNE - 1/2 HP Sump Pump with Integrated Vertical Float Switch and 12 Volt Battery Back Up Capability, Battery Not Included - Up to 5,100 Gallons Per Hour - Heavy Duty Basement Sump Pump System #ad

★★★★☆ 4.5 · 826+ · $570

Amazon

AQUASTRONG 1/2 HP Sump Pump, 3450 GPH Stainless Steel and Thermoplastic Submersible Water Pump, Automatic Float Switch with Piggy-back Plug,1-1/2" NPT Discharge Subpumps for Basement and Flooding Area #ad

★★★★ 4.4 · 78 · $120

Amazon

Westinghouse Battery Backup Sump Pump System – 1/2 HP Dual Sump Pumps with Automatic Switchover, Stainless Steel Construction, Top Suction Design, Basement Flood Protection #ad

★★★★☆ 4.5 · 49 · $570

Amazon

Rainbro 1/2 HP Thermoplastic Submersible Sump Pump With Vertical Float Switch, Utility Pump, Model# PSU051V #ad

★★★★ 4.2 · 228+ · $90

Amazon

LANCHEZ 1/2 HP 5200 GPH Submersible Sump Pump Cast Iron and Stainless Steel with Integrated Vertical Float Switch, Sewage/Effluent Pump for Basement, Basin, Sewage System, 1-1/2" NPT Discharge #ad

★★★★ 4.4 · 108+ · $126

Amazon

Westinghouse Sump Pump 1/2 HP 69 GPM Submersible Water Pump Automatic Vertical Float Switch Heavy Duty Basement Crawl Space Flood Removal Pump High Flow Water Drainage #ad

★★★★ 4.4 · 95 · $110

Amazon

Superior Pump 92057 1/2 HP Thermoplastic Submersible Sump Pump with Vertical Float Switch, plus 1-1/2-Inch Rubber Boot Check Valve #ad

★★★★ 4.4 · 3 · $160

Amazon

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.

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