Battery Backup Runtime Calculator
Runtime note: Runtime is a planning estimate. Battery age, temperature, inverter losses, surge loads, reserve settings, and battery-management limits can shorten real outage time.
Inputs
Result
How to use this calculator
Use this calculator when choosing which circuits or devices should stay on during an outage.
What the result means
The result estimates hours of runtime from usable battery energy and average load.
What the result does not settle
It does not prove the inverter can start every motor load or that the battery can safely deliver every surge.
Inputs that change the answer most
- Usable battery capacity, not just nameplate capacity
- Average watts for all selected loads
- Inverter efficiency
- Reserve state of charge
- Surge power from refrigerators, pumps, compressors, and tools
Readable method
Runtime hours = usable battery energy in Wh ÷ average load in W, adjusted for inverter efficiency and reserve settings.
Before you act
Measure or estimate each critical load separately and confirm inverter surge rating, battery discharge limits, transfer equipment, and manufacturer requirements.
How this is calculated
Load kWh = watts × hours ÷ 1000. Usable AC kWh = battery kWh × DoD × inverter efficiency. Runtime = usable AC kWh ÷ load kW.
How to Use the Runtime Estimate
Use the calculator as a first-pass planning tool for outages, not as a promise that every appliance will run for the displayed time. Enter only the loads you actually expect to keep on during an outage: refrigeration, internet, lighting, medical equipment, well pumps, controls, or a small work area. Large resistance loads such as electric dryers, ovens, space heaters, and water heaters can shorten runtime dramatically and usually belong outside a critical-loads plan.
What Changes the Result Most?
- Usable battery capacity: lithium batteries often allow deeper discharge than lead-acid banks, but the manufacturer limit still matters.
- Inverter efficiency: conversion losses mean a 10 kWh battery does not deliver 10 kWh of AC load.
- Surge loads: refrigerators, pumps, and compressors may start briefly at several times their running wattage.
- Temperature: cold batteries and hot electronics can reduce available performance.
After sizing a rough runtime target, compare it with the battery bank sizing calculator and the inverter sizing guide so energy capacity and instantaneous power are both covered.
Assumptions and formula
Use these inputs as planning assumptions, not as a final design, tax filing, permit package, or equipment approval.
- usable battery energy after reserve settings
- average critical-load watts
- inverter efficiency
- surge loads that may start briefly
- temperature and battery age
Formula
Runtime hours = usable battery watt-hours divided by average load watts, with inverter efficiency and reserve settings kept separate.
Runtime Planning Guide
A backup runtime estimate is most useful when it is tied to a real outage plan. Start by separating must-run loads from convenience loads. A refrigerator, router, modem, medical device, sump pump, boiler controls, a few LED lights, and phone charging may fit comfortably on a modest battery. Electric resistance heat, ovens, clothes dryers, central air conditioning, and large well pumps can consume the same energy much faster and may require load management or a generator.
Example runtime scenario
If a home needs 300 watts for refrigeration and internet, 120 watts for lights and electronics, and an intermittent 600-watt pump that averages 100 watts over time, the planning load is roughly 520 watts. A battery that can deliver 5 kWh of usable AC energy would run that load for about 9.6 hours before safety reserves. In practice, temperature, battery age, inverter losses, and surge events can reduce that number.
Checklist before buying equipment
- Measure or estimate the wattage of each critical load separately.
- Identify surge loads such as pumps, compressors, and refrigerators.
- Confirm that the inverter can start the largest surge load, not just carry the average wattage.
- Decide whether solar, generator charging, or grid recharge will be available during a long outage.
- Keep a manual list of what should be turned off when the system switches to backup mode.
Frequently Asked Questions
Why does runtime drop so quickly when I add one appliance?
Runtime is energy divided by load. A single 1,500-watt heater can draw more power than an entire critical-loads panel, so small additions can cut backup time dramatically.
Should I size for average watts or peak watts?
Use average watts for runtime and peak or surge watts for inverter sizing. A complete backup design needs both numbers.
Sources
Source notes
Use these as starting points when the page affects a purchase, design, tax, utility, or safety decision.