UPS battery runtime calculator

Before a power cut is the wrong time to find out your UPS runs for four minutes. This calculator estimates runtime from the battery bank and the load. Enter the battery voltage, the amp-hours of each battery, how many batteries, how much of the capacity you are willing to use, the inverter efficiency, and the load in watts.

It converts the batteries to watt-hours, takes the usable share after depth of discharge and efficiency losses, and divides by the load. The result is a planning estimate, not a promise.

How the math works

• Battery energy in watt-hours is volts times amp-hours times the number of batteries.
• Usable energy is that figure times the depth of discharge you allow and times the inverter efficiency.
• Runtime in hours is usable watt-hours divided by the load in watts.

Real runtime is shorter when batteries are old, cold or discharged fast, because capacity falls as the draw rises. Lead-acid batteries last longer if you do not drain them deeply, so many people plan for around half or less. Measure your real load with a plug-in meter instead of adding up label ratings.

Worked example

Two 12 V, 9 Ah batteries hold 12 x 9 x 2 = 216 Wh. With 80 percent usable depth and 85 percent efficiency, usable energy is 146.9 Wh. At a 150 W load that is 0.98 hours, about 59 minutes. If the load is only 75 W, the estimate doubles to nearly two hours.