Tools
Estimate how long a UPS battery can carry a load from the battery's stored energy, the inverter efficiency and the load. It gives a first estimate for planning; the maker's runtime table for your UPS and battery is the figure to rely on.
It works out the usable energy in the battery and divides it by the power the inverter draws to carry your load. Stored energy is the number of strings times the blocks per string times each block's nominal voltage and amp-hour capacity; the usable share and the inverter efficiency reduce it; the result, divided by the load, gives minutes. Enter the amp-hour rating from the battery datasheet and the inverter efficiency from the UPS datasheet.
Because real capacity depends on the discharge rate, the temperature and the battery's age. A battery delivers less of its rated amp-hours at the high rates a UPS draws, and less when cold or worn, which is why makers publish runtime tables for each UPS and battery combination. The way to know what an installed string delivers is a capacity test, for which battery makers give a procedure or point to the IEEE one; see battery testing.
Eaton puts a typical UPS battery at five to 15 minutes of backup, with external battery modules or cabinets adding up to several hours at full load. Data-centre UPS are usually sized to carry the load through the generator start with margin; server rooms without a generator need enough for a clean shutdown.
Because it changes both capacity today and life over time. C&D recommends operating at 25 °C within plus or minus 5.5 °C as the best balance of performance and life; colder batteries deliver less, hotter ones age faster. See UPS battery life for the details.
It is an energy estimate. Real runtime is lower at high discharge rates, at low temperature and as the battery ages; use the maker's runtime table to plan and a capacity test to prove it.
On the battery block's label or datasheet. Use the rating at the discharge rate closest to your UPS's, where the datasheet gives several.
The UPS estimates runtime from its own model of the battery and the measured load; after a capacity test or a battery replacement its estimate may need recalibration.
Sources, checked 2026-10-04:
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