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.
- 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
How this calculation works
The bank energy comes from voltage times amp-hours times the number of parallel strings. Three corrections then bring the estimate closer to reality. Only about 80% of the capacity is usable before the unit cuts off to protect the cells. The inverter takes a cut converting DC to mains. And lead-acid batteries give up less than their rated capacity when drained quickly, which a UPS always does, so a Peukert correction is applied against the twenty-hour rating the amp-hour figure assumes. Skip those and the estimate comes out at roughly twice the truth.
What the results mean
- Usable energy
- What the bank can actually deliver to the load after depth-of-discharge limits and inverter losses. Always well below the nameplate watt-hours.
- After ageing
- Sealed lead-acid loses roughly 8% of usable capacity a year in the warm cabinet a UPS lives in. A four-year-old battery is not the battery on the label.
- Load level
- Your load against the unit rating. Above 80%, runtime falls away sharply and the unit has little margin for the surge when equipment restarts.
Common problems and fixes
- The UPS shuts down almost immediately during a power cut
- Test the battery under load rather than trusting the self-test, which typically checks voltage for a few seconds and passes batteries that cannot sustain any real current. Sealed lead-acid is generally finished at three to five years, and the failure is sudden rather than gradual. If the unit is newer than that, check whether the load has grown since it was installed.
- Runtime is much shorter than the manufacturer chart says
- Those charts assume new batteries at 25 degrees, and both assumptions decay. Every 10 degrees above 25 roughly halves battery service life. Check where the unit sits, since a UPS in a warm cupboard or at the bottom of a rack under a server exhaust is cooking its own batteries, and the chart will never match.
Frequently asked questions
What is the difference between VA and watts?
VA is apparent power and watts is real power, and the ratio between them is the power factor. Older UPS units quote a power factor around 0.6, so a 1500 VA unit delivers about 900 W. Size against the watt figure, because that is the one your load actually consumes.
How long should a UPS last during an outage?
Long enough to ride out the short dips, which are most outages, and long enough for connected machines to shut down cleanly. Five to ten minutes covers both if the shutdown is actually configured and tested. Planning to keep working through a long outage means a generator with a UPS in front of it, not a larger UPS.
Can I add more batteries to extend runtime?
Only on units designed for external battery packs, and then only with matched batteries of the same age and type. Mixing old and new cells in a string drags the new ones down to the condition of the old ones, and mismatched capacities in parallel can drive current between strings rather than into the load.
Put this on your own site
Free to embed, no attribution required beyond the source link the frame carries itself. It runs entirely in your visitor's browser, sets no cookies and loads no third-party script.
<iframe src="https://runthetests.com/embed/ups-runtime-calculator/" width="100%" height="560" style="border:1px solid #e5e5e5;border-radius:8px" title="UPS Runtime Estimator" loading="lazy"></iframe>
Preview it at https://runthetests.com/embed/ups-runtime-calculator/. Embedded pages are marked noindex, so yours stays the canonical copy — not this one.