Battery Backup Calculator
How long will a battery run your home during a power cut? Choose your appliances to see the runtime, the load on your inverter, and whether start-up surges could trip it. Use it for home batteries, inverter backups, and portable power stations.
Your backup time
Your battery could run these appliances for about 7 h 38 min.
Assumptions used
- Usable energy
- 3.9 kWh
- Battery draw
- 0.51 kW
- High-current allowance
- 97%
- Solar during outage
- None assumed
Watch the battery drain
It stops at your reserve, which protects the battery and keeps some energy for later.
What if?
Save this result, change any setting, and compare.
How this is calculated
Load
We add up the running watts of the appliances you choose. Fridges and freezers cycle on and off, so we use an average for those.
Load (W) = watts × quantity, added for each appliance
Usable energy
We remove the reserve you keep in the battery, then allow for inverter losses. Batteries deliver less at high current, especially lead-acid, so we reduce capacity using Peukert's law.
Runtime = battery kWh × (1 − reserve) × rate factor ÷ (load kW ÷ inverter efficiency)
Inverter check
The inverter must carry the running load. Motors draw several times their running power when starting, so we also add the largest start-up surge.
Worked example: a 5 kWh lithium battery
Lights, a router, two fans, a television, and a fridge draw about 465 W. With a 20% reserve and a 92% efficient inverter, 3.9 kWh is available.
That runs them for about 7 h 38 min.
How accurate is this?
Runtime is an estimate for a steady load. Real appliances cycle, batteries age, and cold reduces capacity, so plan with a margin. Never rely on this estimate alone for medical equipment. Check with the device maker and keep an independent backup.
Common mistakes
- Using a fridge's start-up power as its average
- Forgetting the inverter's own losses
- Treating all stored energy as available
- Running motors from an inverter sized only for their running watts
Limits of this tool
- Appliance watts are typical values, so check the rating on your own
- It assumes no solar is charging the battery during the outage
- Only one motor is assumed to start at a time
What to do next
- Size a new battery with the battery bank calculator
- See what your panels produce on the solar generation calculator
Frequently asked questions
How long will a 5 kWh battery last?
With the basic home loads in this tool, about 7 h 38 min. The bigger the load, the shorter the runtime, so choose your own appliances above.
Can a battery run an air conditioner?
Yes, if the inverter handles its start-up surge and the battery is large enough. It will drain a home battery far faster than lights and fans.
How do I calculate battery runtime?
Divide usable battery energy by the load, after allowing for reserve and inverter losses.
What size inverter do I need for backup?
It should exceed your running load and handle the largest start-up surge. Check the surge rating on its datasheet.