UPS Runtime Calculator
Estimate UPS backup runtime in minutes from battery watt-hours (or VAh × power factor), load, and inverter efficiency.
Processed locally in your browser — nothing is uploaded.
UPS Runtime Calculator
Sum of battery voltage × Ah for all internal batteries.
~85% is typical for line-interactive UPSes.
Used only to warn about overload.
Results
- Estimated runtime
- 85 min
- Usable energy at the outlet
- 425 Wh
Real UPS runtimes are nonlinear — batteries deliver less energy at high discharge rates, so heavily loaded units run noticeably shorter than this linear estimate, and lightly loaded ones often run longer. Check the manufacturer's runtime chart for critical loads.
About UPS Runtime Calculator
Enter the battery energy in Wh (or VAh with the power factor from the spec sheet), your load in watts, and the inverter efficiency (~85% is typical). The estimated runtime is battery energy × efficiency ÷ load. An optional UPS wattage field warns when the load exceeds what the unit can deliver at all.
Common uses
- Check whether a UPS keeps a router and NAS alive long enough to shut down cleanly
- Compare runtime between a 600 VA and 1500 VA unit for the same PC
- Decide how much load to shed to reach a runtime target
Example
Input
500 Wh battery · 300 W load · 85% efficiency
Output
≈ 85 minutes
Limitations
- Real UPS runtimes are nonlinear — heavy loads drain batteries disproportionately fast, so manufacturer runtime charts beat this linear estimate near full load.
Frequently asked questions
How do I estimate how long a UPS will last in an outage?
Multiply the battery's watt-hours by the inverter efficiency and divide by your load in watts. A 500 Wh battery at 85% efficiency carrying a 300 W load gives roughly 85 minutes.
What is the difference between VA and watts on a UPS?
VA is apparent power and watts is real power; they differ by the power factor, typically around 0.9 for modern equipment. That is why the calculator's VAh mode multiplies battery VAh by the power factor from the spec sheet to get true watt-hours.
Why does my UPS shut down sooner than the estimate?
Battery discharge is nonlinear — heavy loads drain capacity disproportionately fast — and batteries lose capacity as they age. Near full load, the manufacturer's published runtime chart is more trustworthy than any linear estimate.
Is any of my setup information sent off my device?
No. The runtime math runs in your browser and the battery, load, and rating figures you enter are never transmitted or retained.
Can a UPS run a load bigger than its wattage rating?
No. If the load exceeds the UPS's rated output, it will trip into overload or shut down regardless of how much battery is attached — the calculator warns you when your load crosses the rating you enter.