UPS Battery Backup Time Calculator
How long your UPS or inverter battery will keep fans, lights, a router or a fridge running during load shedding.
Add up what stays on: a fan is 60 to 80 W, an LED light 10 to 20 W, a router 10 W, a TV 80 to 120 W, a fridge about 150 W.
For batteries wired in series or parallel; the total stored energy is what counts.
Backup time
3 h 24 min
- Energy stored
- 2,400 Wh
- Usable energy
- 1,020 Wh
A 200 Ah 12 V lead-acid or tubular battery runs 300 W for about 3 h 24 min.
How it's worked out
- Stored
200 Ah × 12 V × 12,400 Wh - Usable
× 50% depth of discharge × 85% inverter efficiency1,020 Wh - Backup time
1,020 Wh ÷ 300 W3 h 24 min
- Lead-acid batteries last longer when drained only halfway, so this uses 50%. Old batteries hold less.
- Fridges and motors draw more when they start, and a UPS also charges the battery from mains; real backup is often a little shorter.
The formula
Backup hours = Ah × volts × batteries × depth of discharge × inverter efficiency ÷ load watts
Worked example
200 Ah × 12 V = 2,400 Wh stored. Using half of it at 85% efficiency gives 1,020 Wh, which runs a 300 W load for about 3 hours 24 minutes.
Questions people ask
How long will a 200 Ah battery last?
A 200 Ah 12 V tubular battery running 300 W lasts about 3 hours 24 minutes, draining it halfway with an 85% efficient inverter.
Why only use half the battery?
Lead-acid and tubular batteries wear out much faster if drained fully, so 50% is the usual limit. Lithium batteries can safely use about 90%.
How do I work out my load?
Add up what stays on: a fan is 60 to 80 W (a DC inverter fan about 30 W), an LED light 10 to 20 W, a router 10 W, a TV 80 to 120 W.
About this calculator
Formula and rules last checked 25 October 2026Written and tested by the CalculatorDex teamReport a problem