How backup time is calculated
Backup time depends on the load, the battery capacity and how much of that capacity is usable.
The basic formula
Backup time (h) = (Battery Ah × Battery V × Depth of discharge × Inverter efficiency) ÷ Load (W).
Worked example
A 150 Ah, 12 V tubular battery, with 0.5 usable depth of discharge and 0.8 inverter efficiency, running a 200 W load:
Backup = (150 × 12 × 0.5 × 0.8) ÷ 200 = 3.6 hours.
What changes the result
- Higher load shortens backup time.
- Older batteries deliver less capacity than new ones.
- Temperature affects capacity; cold batteries deliver less.
- The depth of discharge you choose trades backup time against battery life.
Use the calculator on the home page to estimate your own setup, then confirm with a dealer for the exact model.