How long will my UPS last?
UPS backup time depends on three things: how much energy the battery holds, how much of it the UPS can use, and how much your appliances draw. Here is how to work it out, step by step.
1. Read your UPS and battery
- VA (on the UPS label) is the UPS's size. It decides how much it can run at once, not for how long. A UPS can supply about 0.8 W for each VA: a 1000 VA UPS runs up to about 800 W.
- Ah (on the battery sticker) is the battery's size. Multiply by the voltage to get the energy in watt-hours (Wh): 12 V × 200 Ah = 2400 Wh.
2. Work out the usable energy
A UPS switches off before the battery is completely empty. For a tubular battery that is at about 80% of its capacity: 2400 Wh × 80% = 1920 Wh.
3. Add up what you run
Add the watts of everything on at the same time. The UPS also loses some energy turning battery power into mains power (it is about 85% efficient) and uses about 20 W itself. For 300 W of appliances: 300 ÷ 0.85 + 20 = 373 W from the battery.
4. Divide
1920 Wh ÷ 373 W = 5.1 h. That is the backup from a full battery.
Why the real answer is often shorter
That calculation assumes the battery starts full. With daily load-shedding it often does not: the grid is not on long enough to refill it between cuts. In a typical home on 6 h a day, 1 h every 4 h, a full battery would give 3.9 h, but after a few days each cut gets only 47 min. Read why your UPS backup keeps getting shorter.
Things that also shorten backup
- Fridges, water motors and other motors need a burst of power to start; a small UPS may not manage it while other things run.
- Heavy loads drain a lead-acid battery faster than the simple sum suggests.
- Old and hot batteries hold less.
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