Battery Backup Calculator
Work out how long a battery will run your essential loads during an outage.
Please note: Estimates for planning only. Actual solar output depends on your latitude, roof orientation, shading, weather and equipment. Get a site-specific assessment and quotes before committing.
What the Battery Backup Calculator does
Backup runtime is usable battery energy divided by the load. Usable energy is lower than the nameplate capacity because batteries reserve charge to protect cell life, and the inverter loses a further slice converting DC to AC.
Formula
Usable energy = Capacity × Depth of discharge × Inverter efficiencyRuntime hours = Usable energy ÷ (Load watts ÷ 1,000)With solar: Days = Usable energy ÷ (Daily consumption − Daily recharge)
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Battery capacity | kWh | Yes | Accepts more than 0. |
| Usable depth of discharge | % | Yes | — |
| Inverter efficiency | % | Yes | — |
| Essential load given as | one of 2 options | Yes | — |
| Essential load | W | In some modes | Accepts more than 0. Shown Essential load given as is Total watts. |
| Appliance wattages | text | In some modes | One value per appliance running during the outage. Shown Essential load given as is List of appliance watts. |
| Daily solar recharge | kWh | Optional | If solar keeps working during the outage. Accepts 0 or more. |
How to use it
- Choose Essential load given as.
- Enter Battery capacity, Usable depth of discharge and Inverter efficiency.
- Fill in the remaining inputs the form shows for your choice.
- Optionally add Daily solar recharge.
- Select Calculate.
Worked example
A 13.5 kWh battery at 90% depth of discharge, 92% inverter, running a 750 W essential load.
- Capacity
- 13.5 kWh
- DoD
- 90
- Inverter
- 92
- Load
- 750 W
Usable 11.18 kWh ÷ 0.75 kW = 14.9 hours, or about 0.62 days.
Frequently asked questions
What should count as an essential load?
Fridge, lighting, internet, phone charging and any medical equipment. Heating, cooking and air conditioning are large enough to drain a home battery within hours.
Will my solar keep working in an outage?
Only with a hybrid inverter and the right switchgear. A standard grid-tied system shuts down during an outage for the safety of line workers.