UPS Runtime Calculator
Estimate how long a UPS or battery backup keeps your equipment running, and the VA size to look for.
Backup runtime
1:21:36
- Minutes
- 82
- Load in watts
- 300 W
- VA to look for
- 417
The UPS should run this load for about 82 minutes.
Calculation breakdown
| Step | Formula | Result |
|---|---|---|
| Usable energy | 600 Wh × 80% × 85% | 408.0 Wh |
| Runtime | 408 Wh ÷ 300 W | 1:21:36 |
How it works
Enter the load, the battery energy and, if you know them, the usable share and efficiency.
Worked example
A 300 W load on a 600 Wh battery at 80% usable and 85% efficiency:
- Usable energy: 600 × 0.80 × 0.85 = 408 Wh
- Runtime: 408 ÷ 300 = 1.36 hours (81.6 minutes)
- VA to look for: 300 ÷ 0.9 × 1.25 = 417 VA
Sources
Frequently asked questions
How do you calculate UPS runtime?
Multiply the battery watt-hours by the usable share and the inverter efficiency, then divide by the load in watts. A 600 Wh battery at 80% usable and 85% efficiency runs a 300 W load for about 1.36 hours.
What is the difference between VA and watts?
VA is the apparent power and watts is the real power, and watts equal VA times the power factor. A load of 500 VA at a power factor of 0.9 draws 450 W.
Where do I find the battery watt-hours?
Multiply the battery voltage by its amp-hour rating, for example 12 V times 50 Ah is 600 Wh. If the UPS has several batteries, add them up as the series and parallel layout requires.
Why are usable share and efficiency examples?
UPS makers publish runtime charts for their own models, and those could not be opened to cite here. The 80% and 85% defaults are examples you should replace with your UPS data.
What VA rating should I buy?
Take your load in VA and add headroom so the UPS is not at its limit. The calculator uses 25% as an example that you can change.
An estimate for information only. A real UPS runtime changes with battery age and load; use the maker's runtime chart for your model.