Sump pump sizing Last reviewed June 2026
Sump Pump Battery Backup Sizing + Runtime Chart
By the WhatSize editorial team 4 min read Standards-reviewed
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.
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
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.
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 Battery Backup: What Size Do You Need?
- Sump Pump Installation Guide: Pipe Size, Check Valves & More
- Sump Pump GPH Chart: Capacity by Horsepower & Lift