UPS & Inverter Backup Calculator
How long will your UPS battery last? Or what battery size do you need for load-shedding hours?
Runs in your browser — your data never leaves your device.
About the UPS & Inverter Backup Calculator
Planning for load shedding? Add up the watts of what you want to run — fans, lights, router, TV — and find out how long your UPS battery will last, or which battery you need for the hours of backup you want.
The usable capacity setting matters: lead-acid and tubular batteries last many more years if you use only part of their capacity each time, while lithium batteries can safely use about 90%.
How to use the UPS & Inverter Backup Calculator
- Choose Backup time or Battery size needed.
- Enter the total load in watts (check appliance labels).
- Enter the battery details or the hours you need.
- Adjust usable capacity and inverter efficiency if you know them, then click Calculate.
Formula
Backup hours = battery Ah × volts × number of batteries × usable % × inverter efficiency ÷ load watts.
Battery needed: Wh = load × hours ÷ (usable % × efficiency); Ah = Wh ÷ volts.
Examples
- 400 W load on one 200 Ah 12 V battery (80% usable, 85% efficient) → about 4 h 5 min.
- 400 W for 4 hours → about 2.35 kWh, roughly a 200 Ah 12 V battery.
Frequently asked questions
How accurate is this?
It is a planning estimate using standard formulas. Real results depend on the equipment, installation and conditions, so confirm with a qualified installer before buying.
Why is my real backup shorter?
Older batteries hold less charge, fans and fridges draw more at start-up, and heat reduces capacity. Test your system and keep some margin.