UPS calculator: calculate UPS size in VA and runtime

The required apparent power in VA with a safety margin and the next standard UPS size, or the backup runtime from the battery data, each with calculation steps. Example: A 450 W load at cos φ 0.9 requires 625 VA with 25% reserve, so you need a 650 VA UPS.

Total active power of all devices in watts ÷ cos φ = apparent power in VA. Usually, you then add 20 to 30 % headroom. The calculator rounds up to the next standard UPS size between 400 and 20,000 VA: 450 W at cos φ 0.9 with 25 % headroom gives 625 VA, so you need a 650 VA UPS. The UPS must support both the VA rating and the wattage; both are listed in its datasheet.

Watts measure the active power a device uses; VA measure the apparent power the UPS must supply. P = S × cos φ, or S = P ÷ cos φ: a 1000 VA UPS delivers at most 600 W at cos φ 0.6, or 900 W at cos φ 0.9.

Use the value from the datasheet or your exercise. Modern server power supplies range from 0.9 to 0.99; older devices and many exam questions use 0.6 to 0.8. In the calculator, click to choose 0.6, 0.7, 0.8, 0.9, or 1, or enter your own value. The default is 0.9.

Runtime, also called backup time: Ah × V × number of batteries gives the watt-hours. Multiply by efficiency for usable energy, divide by the load in watts for hours, then multiply by 60 for minutes. Two 12 V, 9 Ah batteries last about 26 minutes at 90 % efficiency with a 450 W load.

The VA rating alone doesn't tell you. It only limits the power output; runtime depends on the batteries (Ah, voltage, and number) and the actual load. Enter both in Runtime mode to calculate the minutes.

In Runtime mode, enter a target time in minutes. The calculator then shows the energy needed in Wh (load × minutes ÷ 60 ÷ efficiency) and the capacity in Ah, calculated by dividing by the total voltage of the batteries in series.

The calculation is linear and does not account for the Peukert effect, aging, or minimum battery voltage. Lead-acid batteries deliver less than their rated capacity at high loads and lose performance in the cold. For planning, use the runtime curve in the datasheet.

The IEC 62040-3 standard defines three classes: VFD (offline) switches over only during a power outage, VI (line-interactive) also compensates for voltage fluctuations, and VFI (online) continuously supplies devices through the inverter.

No. The calculator runs in your browser without sign-up. Your values only appear in the URL so you can share the calculation by link.