UPS Runtime and Load Calculator
Estimate UPS load, rated watt capacity and idealized battery runtime from VA, power factor, battery voltage and amp-hours. Real runtime depends on the battery and discharge curve; check the maker's chart.
Result
| Rated watt capacity | 1,350 W |
|---|---|
| Load | 37.0% |
| Nominal battery energy | 432 Wh |
| Idealized runtime | 44.1 minutes |
Note: This is an energy estimate, not a manufacturer runtime curve. Battery age, temperature, inverter self-consumption, high discharge rate, battery chemistry, and cutoff voltage can materially reduce runtime. Use the UPS maker's runtime chart for purchasing decisions.
Learn more
How it works
A UPS has two limits. The watt rating (VA x power factor) is the most load it can carry at all. The battery energy (voltage x amp-hours, in watt-hours) decides how long it can carry that load.
Runtime here is usable battery energy divided by the load: watt-hours x efficiency / load watts x 60 gives minutes. Efficiency covers the loss converting battery DC to AC; 85-90% is typical for small units.
This is an idealized figure. Batteries deliver less energy at high discharge rates, when cold, and as they age, so a real UPS at high load often runs well short of this number. Use it to compare options and sanity-check, then confirm with the maker's runtime chart.
Worked example
A 1500 VA UPS with a 0.9 power factor and a 24 V, 18 Ah battery bank, carrying a 500 W load:
| Watt capacity | 1500 x 0.9 = 1,350 W |
|---|---|
| Load | 500 / 1,350 = 37.0% |
| Battery energy | 24 x 18 = 432 Wh |
| Usable at 85% | 432 x 0.85 = 367.2 Wh |
| Idealized runtime | 367.2 / 500 x 60 = 44.1 minutes |
What is the difference between VA and watts?
VA is apparent power; watts are the real power your equipment uses. The power factor links them. Check your load in watts against VA x power factor.
Where do I find the battery voltage and Ah?
On the battery label or the UPS spec sheet. A 24 V bank is often two 12 V batteries in series; the Ah stays the same as one battery.
How much headroom should I leave?
Many admins keep steady load under about 80% of the watt rating so startup surges and added equipment do not trip the UPS.
Does doubling the load halve the runtime?
Not quite. The formula assumes it does, but lead-acid batteries deliver more of their energy at lower discharge rates, so real runtime beats the formula at light load and falls short at heavy load.
Maintained by the RACKCRUNCH team. Last reviewed September 29, 2026. How we calculate | Report a problem