UPS Runtime Calculator
Estimate how long a UPS will run your load, size the VA rating you need, and apply Peukert derating so the figure reflects real battery behaviour rather than the marketing number.
Last reviewed by the Radiatus Cloud team
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Why the box never matches reality
A UPS advertises a VA rating and a runtime chart, and both are measured under conditions your rack does not match. VA is apparent power; the watts your equipment actually consumes are lower by the power factor, and modern servers with active power factor correction sit near unity while older gear does not. Meanwhile the runtime chart assumes new batteries at twenty five degrees, a specific discharge profile and a cutoff voltage that leaves the battery deeply discharged.
Batteries deliver less when you pull harder
Lead acid capacity is rated at a twenty hour discharge. Pull the same energy out in ten minutes and you get substantially less than the amp hour label suggests, because internal resistance and the Peukert effect reduce the usable capacity as current rises. A 7 amp hour battery bank supporting a load that draws twenty amps will not deliver twenty one minutes. Ignoring this is the reason a UPS sized on paper shuts down before the generator starts.
Sizing with headroom that means something
The calculation here takes your real watt load, applies inverter efficiency to get the DC draw, converts to battery current at the bank voltage, and applies a Peukert exponent appropriate to the chemistry. It then reports runtime at full and reduced load, the VA rating you should be shopping for, and how the estimate degrades as the battery ages, because a three year old battery string commonly holds only sixty to eighty percent of its original capacity.
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Frequently Asked Questions
What is the difference between VA and watts?
VA is voltage times current without regard to phase; watts is real power. For equipment with a power factor below one, watts are lower than VA. A UPS is limited by both, so size against whichever constraint you hit first, usually watts for modern IT loads.
Why is my real runtime shorter than the chart?
Three reasons stack up: the Peukert effect reduces usable capacity at high discharge rates, inverter efficiency costs another ten to fifteen percent, and battery capacity falls with age. The calculator applies all three so the number is closer to what you will observe.
How much headroom should I leave?
Sizing to about seventy to eighty percent of the UPS watt rating is common practice. It leaves room for load growth and keeps the unit out of the region where efficiency falls and the fans run constantly.
When should batteries be replaced?
Sealed lead acid strings in a UPS typically hold usable capacity for three to five years, less in a warm room. Every ten degrees above twenty five roughly halves service life. Runtime testing under real load is the only reliable check.
Does this work for lithium UPS units?
The runtime maths applies, but lithium iron phosphate cells have a much flatter discharge curve and a Peukert exponent close to one, so the derating is far smaller. Select the lithium option and the calculation reflects that.
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How to Use
Enter your load in watts and the UPS battery specification to estimate realistic runtime.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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