Battery Backup Calculator: Inverter and UPS Backup Time
Enter the battery Ah and voltage, how many batteries, and the load you will run. You get backup time in hours and minutes, with stored and usable energy.
- Nothing you type is stored
- Free
- No signup
Backup time
4 h 05 min
- In hours
- 4.08 h
- Stored energy
- 1,800 Wh
- Usable energy
- 1,224 Wh
- Battery current
- 29.4 A
Estimate only. Real results depend on your equipment, its age and conditions.
Batteries in series add voltage, in parallel add Ah; stored energy is the same either way.
Show working
- 150 Ah × 12 V × 1 = 1,800 Wh
- 1,800 × 0.8 × 0.85 = 1,224 Wh
- 1,224 ÷ 300 W = 4.08 h
How to use
- Enter battery capacity (Ah) and voltage (usually 12 V).
- Enter how many batteries and the total load in watts.
- Adjust inverter efficiency and usable depth under More options.
Good to know
Backup (hours) = Ah × volts × number of batteries × usable depth × inverter efficiency ÷ load (W). A 150 Ah 12 V battery with an 85% efficient inverter, using 80% of its charge, runs 300 W for about 4 hours.
Usable depth: lead-acid and tubular batteries last longer if not drained below about 20–50% left, so 50–80% usable is common; lithium batteries usually allow 80–90%. Inverter efficiency is typically 80–90%.
At heavy loads lead-acid batteries deliver fewer Ah than their rating (Peukert effect), and old batteries hold less. Ratings are usually at the 10 or 20 hour rate (C10/C20), so real backup at high loads can be noticeably shorter.
Questions
How long will a 150 Ah battery run 3 fans and 4 lights?
About 300 W of load: roughly 4 hours with an 85% inverter and 80% usable charge.
Does two batteries double the backup?
Yes, at the same load, two 150 Ah batteries in a 24 V system store twice the energy of one.
What is usable depth of discharge?
How much of the stored charge you use before recharging. Deeper discharge shortens lead-acid battery life.
Why is my real backup less?
Battery age, high loads, low inverter efficiency and incomplete charging all reduce backup.
How many watts can a 12 V 150 Ah battery give?
Energy is 1,800 Wh. The power limit is set by the inverter; a 900 VA inverter gives about 700 W.