UPS Runtime Calculator

How it works

runtime = battery Ah × V × count × 80 % × inverter efficiency / load W
load W  = load VA × power factor

A UPS has two ratings: VA (apparent power) and W (real power). The watt rating is the one your load must stay under; 80 % of it leaves headroom. Modern computer power supplies have a power factor close to 1, so their VA and W are nearly equal.

Small UPS units use sealed lead-acid batteries discharged at a high rate, where they deliver well under their nominal capacity. Treat this as an upper-bound estimate and compare with the manufacturer’s runtime chart, which is measured.

References

• Uninterruptible power supply — Wikipedia