UPS Runtime Calculator
Estimate UPS battery backup runtime in minutes from capacity (VA or Wh), connected load, and UPS efficiency.
Assumes a typical power factor of 0.8 to convert VA to watt-hours. Choose Wh instead if your UPS documentation already states capacity in watt-hours.
W
%
Results
Estimated Runtime
192.0 min
Runtime in Hours
3.20 hours
Usable Energy
1200 Wh
How Does the UPS Runtime Calculator Work?
Enter your UPS's rated capacity (either in VA — volt-amps — or Wh — watt-hours, whichever your unit's label or manual specifies), the power draw of the equipment you have plugged into it, and the UPS's efficiency. If your capacity is in VA, the calculator applies a typical power factor of 0.8 to estimate the usable watt-hours, since VA and watts aren't the same unit for most UPS loads. It then multiplies the usable energy by the efficiency and divides by your connected load to estimate runtime in hours and minutes.
Example
A 1,500 VA UPS at a 0.8 power factor provides about 1,200 Wh of usable energy. Running an 80% efficient UPS with a 300W connected load (a desktop PC and monitor), the estimated runtime is (1,200 × 0.8) ÷ 300 = 3.2 hours, or about 192 minutes.
Common Use Cases
- Sizing a UPS to keep a server or network equipment running through a short power outage.
- Checking how long a UPS will bridge power for a home office setup.
- Comparing runtime across UPS models with different VA/Wh ratings.
- Planning how much load a UPS can safely support for a target backup duration.
FAQs
Why does VA capacity need a power factor to estimate watt-hours?
VA (apparent power) and watts (real power) are only equal when the power factor is 1.0, which is rare for real equipment. Most UPS units and the loads they power have a power factor around 0.6-0.9, so this calculator uses 0.8 as a reasonable estimate — check your UPS's documentation for its exact rated watts if you need a more precise figure.
What efficiency should I use for my UPS?
Standard offline/line-interactive UPS units are commonly rated around 80-90% efficient in normal (non-battery) operation; online double-conversion units can run somewhat lower due to constant AC-DC-AC conversion. Check your unit's spec sheet, or use 80% as a conservative default.
Is this runtime estimate exact?
No — it's a simplified estimate. Real UPS runtime also depends on battery age and health, temperature, and the specific discharge curve of the battery chemistry, all of which can shift actual runtime below a brand-new battery's rated figures.