UPS Runtime Estimator

Estimate how long a UPS really holds a load, batteries and losses included.

What you provide

Read it from the UPS display or a plug-in power meter. Do not use the sum of power supply ratings.

The watt figure, not the VA figure. Watts are usually 55% to 70% of VA.

Result

Roughly 15 min at 300 W

Runtime falls away faster than load rises, so halving the load buys more than double the time. What a UPS is genuinely for is riding out the short dips and giving the machines long enough to shut down cleanly, and five minutes is enough for that if the shutdown is actually configured. If the plan is to keep working through an outage, that is a generator with a UPS in front of it, not a bigger UPS. Batteries are consumables: sealed lead-acid units are usually done at three to five years, and the failure mode is a unit that reports healthy until the moment it is asked to work.

Diagnostic telemetry
Load33% of the unit's 900 W rating
300 W
Battery bank24 V × 9 Ah × 1
216 Wh
After ageing2 years old
84% of new
Usable energy80% depth of discharge, after inverter losses
131 Wh
Estimated runtime
15 min
Load level
light

What this cannot tell you

  • An estimate from the ratings you enter. It cannot measure your batteries, and a battery that reports healthy can still fail the moment it is asked to work.
  • The only reliable runtime figure comes from a real load test on your own unit. Battery chemistry, temperature and age all move the answer, and this arithmetic assumes sealed lead-acid at room temperature.

Take this with you

UPS Runtime Estimator on RunTheTests — free browser-based sysadmin tools, no sign-up.