Estimate continuous inverter output, total startup/surge output, battery capacity, and modeled runtime or cycle equivalents for an AC sump pump powered through an inverter. Start with the pump’s real nameplate, manual, or measured values whenever possible.
Estimate only—not an installation guide. Test the real pump and backup together. This tool does not provide permanent wiring, transfer-switch, or electrical-code guidance.
Your pump and outage plan
Planning estimate
Backup requirements
Running value: . Startup value: .
Compatibility caveat: Watts alone do not prove inverter current/VA, waveform, surge-duration, or motor-start compatibility. Check both manufacturers’ limits and test repeated starts safely.
Check these limiting factors
How the math works
The calculator keeps power (watts) separate from energy (watt-hours), and keeps continuous output separate from total startup/surge output.
- Duty cycle = minutes per cycle / 60 * cycles per hour, capped at 100%.
- Active run hours = duty cycle * outage hours.
- Pump AC energy (Wh) = running watts * active run hours.
- Battery-side energy before reserve = pump AC energy / inverter efficiency + inverter idle watts * idle hours.
- Recommended battery nameplate Wh = battery-side energy / usable depth of discharge * (1 + reserve margin).
- Minimum continuous output = running watts * reserve factor.
- Minimum total startup/surge output = total startup watts * reserve factor. An “additional starting watts” figure must first be added to running watts if its source defines it that way.
- Known-battery runtime = battery nameplate Wh * usable DoD / reserve factor / average battery-side load.
Solar note: use only net battery-side charging power averaged across the entire modeled interval, including darkness and conversion losses. It is an illustrative runtime scenario and never reduces minimum recommended battery capacity.
Cycle counts are modeled energy equivalents from the average schedule—not guaranteed completed pumping events.
Read the full 1/2 HP sump-pump sizing guide for context and equipment-selection limits.
Defaults are illustrative, editable planning assumptions—not product or pump specifications. If startup watts are omitted, total startup demand is screened at 3.5 * running watts, based on the approximate ratio in a published Reliance transfer-switch load-estimating example for a 1/2 HP sump pump. Your actual nameplate, manual, manufacturer data, or suitable measurement wins.
Safety scope
- This is an estimate, not a compatibility guarantee or flood-protection plan.
- Use actual pump/manual/nameplate values and test repeated starts with the real system under safe conditions. Do not run a pump dry.
- Check voltage, current/VA capability, waveform, grounding, GFCI, environment, surge duration, motor-start support, and automatic-restart behavior in both manufacturers’ instructions.
- No permanent wiring, transfer-switch, backfeed, or code guidance is provided here. Use a licensed professional for permanent electrical work.
- Keep battery equipment dry, ventilated as required, accessible, and above potential floodwater.