Inverter battery backup time calculator
See how many hours your battery will keep things running, after allowing for the usable share of the battery and inverter losses.
Runs only on the figures you enter. Nothing you type leaves your browser.
Your details
Backup time
10.13 hours
Covers a 4-hour outage with room to spare.
How we worked it out
- Battery capacity5.0 kWh
- Usable energy4.1 kWh
After 90% depth of discharge and 90% inverter efficiency
- Load400
watts
- Backup time10.13 hours
The formula
Backup hours = battery kWh × usable share × inverter efficiency × 1 000 ÷ load in watts.
Tips for longer backup
- Put only essentials on the backup circuit: lights, Wi-Fi, TV, chargers and the fridge.
- Check the inverter's continuous power rating too. A big battery cannot run a load larger than the inverter allows.
- Batteries lose capacity with age and in the cold, so allow a margin.
Frequently asked questions
How long will a 5 kWh battery last?
Running 500 W, a 5 kWh lithium battery lasts about 8 hours (5 kWh × 90% usable × 90% inverter efficiency ÷ 0.5 kW). Double the load and the time halves.
What can I run on a small inverter?
Wi-Fi, TV, laptops, phone chargers and LED lights use little power. Kettles, heaters, geysers, ovens, irons and hair dryers use a lot and drain a small battery in minutes; avoid them on backup.
How do I convert amp-hours to kWh?
Multiply volts by amp-hours and divide by 1 000. A 12 V 200 Ah battery is 2.4 kWh; a 48 V 100 Ah battery is 4.8 kWh.