UPS Runtime Calculator
Backup minutes from battery and load.
What it does
UPS runtime answers how long the beeping lasts: stored watt hours, trimmed to usable share and inverter efficiency, divided by load watts. Nameplate energy alone always overpromises.
The tool above takes battery Wh with usable share, efficiency, and load. It returns runtime with usable energy beside it, in hours and minutes.
The defaults (1000 Wh lead-acid at 50%, 90% efficient, 200 W load) run 2.25 h, shown as 2 h 15 min. Lithium at 85% stretches the same pack to 3.83 h.
Efficiency practice follows ENERGY STAR UPS criteria with DOE purchasing guidance matching: topology classes, load weighted efficiency, and sizing near real loads. Usable shares stay labeled as design claims, not certified specs.
How to use
- Enter battery nameplate Wh.
- Pick lead 50% or lithium 85% usable.
- Enter inverter efficiency.
- Enter total load watts.
- Read runtime first.
- Derate old or hot batteries.
Formula
- Wh
- Nameplate battery energy.
- share
- Usable fraction: 50% lead, 85% lithium.
- eff
- Battery to AC inversion efficiency.
- load
- Protected watts drawing together.
ENERGY STAR normal mode figures near 98% describe grid passthrough, not battery runtime inversion; this page uses the separate 90% style assumption you can edit. Peukert effect, age, heat, and high discharge rates all shrink real watt hours.
Worked example
Example: 1000 Wh lead, 200 W load
Defaults: 1000 Wh, lead 50%, 90% efficient, 200 W.
- Usable: 1000 x 0.5 = 500 Wh.
- Runtime: 500 x 0.9 / 200 = 2.25 h.
- Minutes: 0.25 x 60 = 15, so 2 h 15 min.
- Tool shows 2.25 h (2 h 15 min), 500 Wh usable.
- Lithium 85% gives 3.83 h instead.
- Old packs derate further from here.
Reference table
| Setup | Usable | Runtime |
|---|---|---|
| Lead, 200 W | 500 Wh | 2.25 h |
| Lithium, 200 W | 850 Wh | 3.83 h |
| Lead, 500 W | 500 Wh | 0.9 h |
Limits
- Estimate, never a shutdown plan alone.
- Age and heat cut capacity.
- High rates drain faster (Peukert).
- VA rating separate from watts.
Common mistakes
- Using nameplate Wh as usable.
- Mixing grid efficiency with runtime.
- Forgetting inverter losses.
- Overloading past VA rating.
- Trusting old batteries at spec.
Try it yourself
Quick checks. Answers sit tucked under each question.
Lithium 85% on defaults?
850 Wh usable, 3.83 h runtime.
Lead at 500 W load?
500 Wh usable, 0.9 h runtime.
Why derate old packs?
Age, heat, and cycles shrink real watt hours.
Questions
How long will my UPS last?
Usable Wh times efficiency over load watts.
Lead vs lithium runtime?
Lithium uses ~85% vs ~50% lead, so longer per nameplate.
What efficiency to use?
90% is a fair runtime default; check your unit.
Watts vs VA?
Watts do work; VA includes reactive power. Respect both ratings.
Bigger load effect?
Runtime falls linearly with load, faster with rate losses.
Shutdown planning?
Set auto shutdown well inside estimated runtime.