Inverter Battery Backup Calculator
How long your inverter battery will last on your load
Your Backup Time
Backup at different loads
| Load | Backup |
|---|---|
| 100 W | 12 h 14 min |
| 200 W | 6 h 7 min |
| 300 W | 4 h 5 min |
| 500 W | 2 h 27 min |
| 750 W | 1 h 38 min |
| 1000 W | 1 h 13 min |
- 1Enter your batteryAh, voltage and how many
- 2Add your loadWatts of everything on the inverter
- 3See the backup timeOr the Ah you need
How is inverter backup time calculated?
Backup is the battery's usable energy divided by the load, after the inverter's own losses. You shouldn't fully drain a battery, so only part of its capacity counts.
Backup (hours) = Ah × V × batteries × usable share × efficiency ÷ load (W)
Battery Ah needed = hours × load ÷ (V × batteries × usable share × efficiency)
Add up your load
Typical wattages: ceiling fan 50–75 W (BLDC fans about 30 W), LED bulb 9–12 W, LED TV 60–100 W, Wi-Fi router 10 W, laptop 50 W, fridge 100–200 W running. Avoid running irons, geysers, ACs or pumps on a home inverter.
Battery life
Lead-acid and tubular batteries last longer if they aren't drained below about 20%. Top up distilled water in flooded tubular batteries every 2–3 months, and keep them in a ventilated place.
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Frequently asked questions
How long will a 150Ah battery last?
On a 300 W load, about 4 hours (150 × 12 × 0.8 × 0.85 ÷ 300). On 100 W, about 12 hours.
Why is my backup less than calculated?
Older batteries lose capacity, and heavy loads drain lead-acid batteries less efficiently. Inverter idle consumption and loose connections also cut backup.
Which inverter rating do I need?
Add up the load in watts and choose an inverter whose VA rating is about 1.25–1.5 times that. A 300 W load suits a 600–900 VA inverter.
Last reviewed: October 2026