UPS Runtime Calculator
Size a UPS in VA and watts for your equipment and estimate battery runtime from the battery voltage, amp-hours and inverter efficiency, with a lead-acid rate correction.
The rate-adjusted estimate assumes the batteries are wired in series (n × 12 V), as in most UPS units.
Buy a UPS whose VA and watt ratings both reach the figures above: many 1000 VA units are rated only 600–700 W.
The rate-adjusted runtime uses Peukert's law (exponent 1.2) because batteries deliver less energy at high current; the maker's runtime chart for your model is the best guide.
A UPS must handle your equipment's power with room to spare and keep it running long enough to save work or ride out a short cut. This calculator sizes the UPS in volt-amperes and watts from your load, power factor and growth headroom, and estimates runtime from the battery bank — once as the ideal energy-based figure and once corrected for the capacity lead-acid batteries lose at high current.
How to use this tool
- Enter the total load in watts, or list each device with its watts.
- Keep the power factor at 0.9 for modern PCs and add headroom for growth.
- Enter the battery voltage, amp-hours and number of batteries (from the battery label or the UPS manual).
- Read the UPS size and the runtime estimates.
Formula
VA = W ÷ PF × (1 + headroom) · ideal runtime = V × Ah × n × efficiency × DoD ÷ W · rate-adjusted t = 20 h × (Ah ÷ (I × 20 h))^k
I is the battery current (load ÷ efficiency ÷ battery-string voltage) and k the Peukert exponent: about 1.2 for sealed lead-acid batteries rated at the 20-hour rate, close to 1 for lithium. UPS batteries are discharged at high rates, so real runtimes are well below the ideal figure.
Example
A 300 W load on 2 × 12 V 9 Ah batteries (216 Wh) at 90% inverter efficiency gives an ideal 216 × 0.9 ÷ 300 = 0.648 h ≈ 38.9 minutes; the lead-acid estimate is about 20 minutes. A 600 W load at power factor 0.9 with 25% headroom needs 600 ÷ 0.9 × 1.25 = 833 VA and 750 W — a 1000 VA unit with at least a 750 W rating.
Frequently asked questions
What is the difference between VA and watts?
Watts are real power; VA is voltage times current. They differ by the power factor. A UPS has a limit for both, so check that its watt rating covers your load too.
Why is the real runtime shorter than the ideal one?
Lead-acid batteries deliver much less of their rated capacity when drained in minutes instead of 20 hours, and the inverter and electronics lose some power. Ageing batteries lose more.
How long should a UPS last?
Long enough to save work and shut down cleanly — 5 to 15 minutes for a PC — or to bridge to a generator. For longer outages consider extended battery packs or a lithium UPS.
Last updated:
Page address with these inputs: https://kakao.mobile/tools/ups-runtime-calculator/
Cite or embed this tool
kakaoCalc. (2026, November 8). UPS Runtime Calculator. https://kakao.mobile/tools/ups-runtime-calculator/
“UPS Runtime Calculator.” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/ups-runtime-calculator/.
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