UPS Runtime Calculator
How long the box holds the wall up — a UPS's volt-amp label into real watts, then the battery's minutes against your load.
UPS Runtime Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Linear runtime — Peukert and aging not modeled
- No transfer-time or charger-refill behavior
In short: A 1,500 VA desktop UPS delivers about 1,350 W once its 0.9 power factor is paid. The little 9 Ah, 12 V battery inside stores 108 Wh, roughly 92 of them usable through an 85% inverter — so at a 600 W workstation the wall comes back in about 9 minutes 11 seconds. Run the full 1,350 W rating instead and the same battery dies in about 4 minutes 5 seconds. The label sells volt-amps; the battery sells minutes, and it is always the smaller purchase.
Formula
W = VA × PF · usable = Ah × V × η · runtime = usable ÷ W
A UPS's volt-amp rating is a ceiling, not a promise of watts: the output power factor turns 1,500 VA into 1,350 real watts. What keeps the load alive is the battery — amp-hours times volts, shaved by the inverter — divided by what you actually draw. The minutes are linear in the load until Peukert and age disagree with the arithmetic.
Worked Example
- Read the VA rating and power factor off the unit.
- Enter the internal battery's Ah and DC voltage.
- Read the minutes at your real load.
Defaults: 1,500 VA at a 90% power factor carrying 600 W on a 9 Ah, 12 V battery at 85% — 9 minutes 11 seconds, and 4 minutes 5 seconds if the load grows to the full 1,350 W. Halve the load and the minutes double.
Strengths & Limits Of This Model
Where this engine is strong
- VA label converted to honest watts on the page
- Full-rating contrast exposes label shopping
Where it stops
- Internal battery specs are often unlisted
Practical Use Cases
Workstation shutdowns
minutes to a clean save
Network closets
router plus NVR hold-up time
UPS shopping
the label against the real load
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
UPS Runtime Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why is my 1,500 VA UPS only good for 1,350 watts?
Volt-amps multiply by the output power factor to become watts. Modern units sit near 0.9; older or cheap boxes lower. The VA figure is the ceiling the inverter can pass, the watt figure is what reaches the plug.
Nine minutes — is that broken?
No, that is the category. Desktop UPS batteries are sized for minutes: enough to save the file and shut down. Whole-hour runtime needs external battery packs, and the arithmetic here is the same with bigger numbers.
How is this different from the battery page?
The battery page starts from the pack and prices your watts; this one starts from a quoted VA rating, converts it to real watts, and contrasts your load with the full-rating clock. Same clock, different question.
Why do minutes double when the load halves?
Because the battery's energy is fixed and runtime is energy divided by power — until Peukert and aging bend the line. Below about a tenth of the box's rating the lead-acid curves pay you back a little extra.
What power factor should I enter?
Check the spec sheet's output PF. Nine tenths is the modern default; entering 100% is honest only for purely resistive loads like heaters.
Does the UPS itself consume from the battery?
Yes — conversion losses and the electronics' own draw. That is the efficiency line: an 85% inverter hands the load 85 cents of every stored watt.
Can I put a bigger battery in?
Externally, often; internally, rarely. Runtime scales with amp-hours times volts, so a 100 Ah external pack on the same 12 V bus turns minutes into hours — if the charger can ever refill it.
What decides the battery's real life?
Depth of discharge, temperature and cycle count. A UPS that trims its load to half draws half as deep and roughly doubles both the minutes and the battery's calendar.