Path-Q Solar
How should I size battery backup for a sump pump?
Answer
Size it for the pump's starting surge, running power, expected operating hours, and the battery system's usable energy and inverter limits. Because sump pumps may cycle unpredictably during storms, include a conservative runtime margin and consider a separate pump or backup plan.
Key points
- The inverter must handle motor startup, not just the pump's running watts.
- Energy use depends on how often and how long the pump runs.
- A high-water event can exceed normal assumptions, especially during heavy rain or power loss.
What it depends on
- Pump horsepower, voltage, running current, and starting current
- Expected duty cycle and outage duration
- Battery usable kWh and inverter surge rating
- Water inflow, drainage conditions, and alternate pump availability
Risks and trade-offs
- Undersizing can cause inverter trips or flooding.
- Oversizing for extreme events increases cost but may still not eliminate property risk.
- Battery backup does not replace pump maintenance, alarm systems, or a secondary pump where needed.
Information you may need
- Pump nameplate and measured starting data
- Observed cycling during severe weather
- Desired runtime and battery specifications
- Flood consequences and backup pump options
Your next step
What to do next
Measure or obtain the pump's starting and running requirements, then have the installer calculate runtime using a conservative storm scenario.
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SQ01748