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Home Battery Backup Runtime Calculator

Estimate how long a home battery can run selected loads during an outage.

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Inputs

kWh
%
W

Your values stay on this device.

Results

Estimated Runtime
Runtime9.32
Usable Energy11.18
Average Load1.2

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Assumptions

  • Average load is steady across the backup window.
  • Solar recharge during the outage is not included unless you reduce average load or add capacity manually.
  • Calculations run locally and are for planning only.

Sensitivity preview

Battery Capacity -10%: Estimated Runtime 8.4h

Battery Capacity Current: Estimated Runtime 9.3h

Battery Capacity +10%: Estimated Runtime 10.2h

Formula: Runtime hours = battery kWh * depth of discharge percent * inverter efficiency percent / average load kW.

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About this Home Battery Backup Runtime Calculator

Battery backup runtime depends on usable stored energy and the loads you keep running. This calculator converts battery kWh into estimated hours of backup power.

Who this calculator is for

  • Homeowners comparing battery backup sizes.
  • Solar shoppers planning outage coverage.
  • Anyone estimating critical-load runtime.

When to use it

  • Estimate how long a battery will run essentials.
  • Compare whole-home versus critical-load backup.
  • Check whether a proposed battery size fits outage goals.

How to use this calculator

  1. Enter battery capacity in kWh.
  2. Enter usable depth of discharge and inverter efficiency.
  3. Enter average load in watts.
  4. Review runtime and usable energy.

Key assumptions

  • Average load is steady across the backup window.
  • Solar recharge during the outage is not included unless you reduce average load or add capacity manually.
  • Calculations run locally and are for planning only.

Accuracy and testing

Review formula sources, default test cases, assumptions, unit handling, rounding rules, and changelog for this calculator.

View accuracy page

How this calculator works

Usable kWh = Battery kWh x depth of discharge x inverter efficiency. Runtime hours = Usable kWh / average load kW.
Runtime hours = battery kWh * depth of discharge percent * inverter efficiency percent / average load kW.

Inputs explained

Battery capacity

Rated battery capacity in kWh.

Depth of discharge

Percent of capacity you plan to use during backup.

Average load

Average watts from critical loads or whole-home loads.

Understanding your results

Runtime is an estimate. Motor starts, HVAC cycles, surge loads, cold weather, battery reserve settings, and solar recharge can change actual runtime.

Example scenarios

Critical loads

13.5 kWh battery at 90% depth and 92% inverter efficiency running 1,200 W lasts about 9.3 hours.

Key Features

  • Instant Calculation
  • Private & Secure
  • Mobile Optimized

Tips & Best Practices

  • Measure actual critical-load consumption when possible.
  • Keep HVAC, well pumps, and large appliances separate if they cycle heavily.
  • Use a lower depth of discharge if your battery reserves energy for protection.

Frequently Asked Questions

Does solar recharge count?
No. This calculator estimates runtime from stored battery energy only. Solar production during an outage should be modeled separately.
Should I use peak or average load?
Use average load for runtime. Peak and surge loads matter for inverter sizing and whether appliances can start.
Are my home energy values stored?
No. The calculator runs locally in your browser.