Home, energy & building
UPS & Battery Backup Calculator
Find out how long your UPS battery will last during load-shedding. Enter the watts you want to run, the battery size and type, and see the backup time and the inverter size you need.
How to use the UPS & Battery Backup Calculator
- Add up the watts of everything you want to run.
- Enter the battery Ah and voltage.
- Choose lead-acid or lithium.
- Read the backup time.
Formula
Example
A 200 Ah, 12 V tubular battery used to 50% at 85% efficiency gives about 1,020 Wh. Running 400 W of fans and lights, that is about 2 hours 33 minutes.
Typical power use
- LED bulb: 10–15 W
- Ceiling fan: 60–80 W (DC or inverter fan 30–40 W)
- LED TV: 60–120 W
- Wi-Fi router: about 10 W
- Laptop: 50–65 W
- Fridge: 150–250 W while the compressor runs
Get more backup
Switch to DC or inverter fans and LED lights, keep only essential loads on the UPS, and keep batteries in a cool, ventilated place.
Read the guide
Related free tools
Frequently asked questions
Why use only 50% of a lead-acid battery?
Draining a lead-acid battery deeply every day shortens its life a lot. Keeping use to about half its capacity makes it last longer.
Is lithium worth it?
Lithium batteries cost more but give more usable capacity, charge faster and last more cycles.
How long does a battery take to charge?
It depends on the charger current. A 200 Ah battery at 20 A takes about 10 hours or more from half charge, longer in practice.
Results are estimates for planning. For tax, legal or religious decisions, confirm with a qualified person. Your inputs stay in your browser and are not sent to Soft2Cloud.