Sump pump sizing Last reviewed June 30, 2026

Sump Pump Battery Backup Sizing + Runtime Chart

By the WhatSize editorial team 4 min read Standards-reviewed

TL;DR

A 1/2 HP sump pump with a 50-70 Ah deep-cycle battery provides about 3-5 hours of continuous runtime at 10 ft of head. For longer outages, pair with a 100W solar panel or upgrade to a dual-battery system with 100+ Ah total.

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

How Battery Backup Sump Pumps Work

A battery backup sump pump system combines a DC pump with a deep-cycle marine battery and a battery charger. When power is available, the charger keeps the battery topped off. When power fails, the DC pump draws from the battery automatically. Most systems also include an alarm that alerts you when the battery is running low.

The key sizing factors are: (1) pump horsepower, (2) total dynamic head (how high the water must be lifted), (3) battery capacity in amp-hours (Ah), and (4) how often the pump cycles on and off. Most residential systems are designed for intermittent use — pumping a few minutes per hour — which extends battery life 3-5x over continuous run estimates.

Top picks for this size

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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

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

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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

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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

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

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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

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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

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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 by Sump Pump Type

1/3 HP pump: 40-50 Ah battery — about 4-6 hours intermittent runtime (~30 min/hr pumping)
1/2 HP pump: 50-70 Ah battery — about 3-5 hours intermittent runtime
3/4 HP pump: 75-100 Ah battery (or dual 50Ah) — about 3-4 hours for heavy water flow
1 HP pump: 100+ Ah battery bank — dual batteries recommended for heavy basement water

Frequently asked questions

How long will a battery backup sump pump run?

A 1/2 HP pump with a 70 Ah battery (like the Basement Watchdog) runs about 3-5 hours of intermittent pumping. In continuous pumping (worst-case storm), maybe 1-2 hours. A dual-battery system doubles this.

What size battery do I need for my sump pump backup?

For most 1/2 HP sump pumps, a group 27 or 31 deep-cycle marine battery (70-100 Ah) is recommended. Don't use car batteries — deep-cycle batteries can handle repeated discharging without damage.

Can I add a battery backup to my existing sump pump?

Yes. Systems like the Basement Watchdog connect inline with your existing AC pump. The backup pump sits in the same pit, slightly higher than the primary pump, and kicks in only when the primary fails or loses power.

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