Tools
UPS runtime calculator
List what is plugged into the UPS, then enter the battery details from its label or data sheet. The chart shows how runtime falls as the load rises.
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Result
Runtime as the load changes
How the UPS runtime calculator works
UPS real power (W) = rating in VA × power factor. Keeping the load below 80% of that figure leaves room for growth and for the inrush current when equipment starts.
Battery current I = load ÷ (inverter efficiency × battery voltage), with an efficiency of 90%. Battery voltage = blocks in series × block voltage.
Runtime uses Peukert's law with an exponent of 1.2, for lead acid batteries with a 20 hour (C20) capacity rating: t = 20 × (C ÷ (I × 20))^1.2 hours, where C is the amp hour capacity of all strings, reduced for battery age. Below the 20 hour current, runtime is taken as C ÷ I.
Expect the real figure within about 30% of the estimate. Temperature, age and the discharge cut off voltage all move it. The UPS's own runtime chart and a timed test are better sources.
Planning estimates. They do not replace a measurement on your own systems.
In practice
Worked example: two 350 W servers, a 300 W storage unit, two 60 W switches and a 40 W firewall draw 1,160 W. On a 3,000 VA UPS with a power factor of 0.9, that is 43% of its 2,700 W rating. With four 12 V 9 Ah batteries the estimate is about 9 minutes, enough for a clean shutdown but not to ride out a long power cut.
Runtime falls faster than load rises: halving the load more than doubles the runtime. Old batteries are the most common reason a UPS fails during a real power cut, so record the installation date and test under load once a year.
Questions about this tool
How much runtime do we need?
Enough for servers and storage to shut down cleanly, usually 5 to 10 minutes, plus time for a generator to start if you have one. Longer runtimes need external battery packs.
Why size by watts and not VA?
Your equipment draws real power in watts. A UPS with a high VA figure but a low power factor can still be overloaded. Check both figures on the label.
How accurate is the estimate?
Within about 30% for healthy lead acid batteries at room temperature. The manufacturer's runtime chart for your exact model is better, and a timed test is best.
When you want an engineer to look at it
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