How to Use the UPS Backup Runtime Calculator
When a power outage or voltage sag hits, knowing how long your UPS (uninterruptible power supply) can keep your computer, server, or network gear running helps you avoid data loss and gives you time to shut down safely. This calculator uses your battery's total capacity, the actual connected load, and the inverter's conversion efficiency to estimate backup runtime.
A UPS battery stores DC power and converts it to AC to run your devices, and some energy is lost as heat in that conversion. Budget UPS units run around 80% efficient, while high-efficiency online-type units stay above 92%. Lower efficiency means less usable backup time even with the same battery capacity.
If your battery is only labeled in Ah (amp-hours), multiply by the battery voltage — usually 12V for home units — to convert to Wh before entering it. For the load, add up the wattage of every device plugged into the UPS. Keeping the load under half the UPS's rated capacity gives the most stable efficiency and the longest battery life, and for mission-critical setups like a server closet, it's safer to size the battery with about 20% more runtime than you actually calculate you'll need.
Frequently Asked Questions
Multiply the battery's total capacity (Wh) by the inverter's conversion efficiency, then divide by the connected load (W) to get the runtime in minutes. A larger load means a shorter runtime.
Check the UPS label or manual for a rating in Wh (watt-hours) or Ah (amp-hours). If it's listed in Ah, multiply by the battery voltage (usually 12V) to convert to Wh.