UPS sizing and runtime calculator
UPS units are rated in VA, but the load is in watts. The gap is the power factor.
Wh
%
%
Runtime at full load
77.8minutes
- UPS rating to buy
- 694 VA
- Minimum VA for the load
- 556 VA
- Runtime at half load
- 155.5 minutes
- Usable battery energy
- 648.0 Wh
Show the working
- Load = 500
- Power factor of the UPS = 0.9
- Battery energy = 720
- Inverter efficiency = 90
- Sizing headroom = 25
- Minimum VA for the load = 500 / 0.9 = 555.556
- UPS rating to buy = 555.556 * (1 + 25/100) = 694.444
- Usable battery energy = 720 * 90/100 = 648
- hours = 648 / 500 = 1.296
- Runtime at full load = 1.296 * 60 = 77.76
- Runtime at half load = 648 / (500/2) * 60 = 155.52
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- VA = watts / power factor
- runtime = usable battery energy / load
Runtime is not linear in load: at half load most UPS units run rather more than twice as long, because the inverter is more efficient.