UPS Sizing and Runtime Calculator

How is a UPS sized?

A UPS has two limits: apparent power (VA) and active power (W). Their ratio is the UPS output power factor. The load must stay below both.

• Apparent power of the load: S = P / load power factor
• VA equivalent of the watt limit: P / UPS power factor
• Required UPS rating: the larger of the two × (1 + spare margin)

Example

A load drawing 3000 W at a power factor of 0.8 is 3750 VA. For a UPS with an output power factor of 0.9 the watt limit asks for 3333 VA, so 3750 VA governs. With a 25 percent margin at least 4687.5 VA is needed: choose the next standard rating up.

Runtime

t = n × V × Ah × k × η / P

n is the number of batteries, V and Ah the voltage and capacity of one battery, η the inverter efficiency and k the usable share of capacity. Two 12 V 9 Ah batteries hold 216 Wh. With k = 0.6 and η = 0.9 the load receives 116.6 Wh, so a 600 W load runs for about 11.7 minutes.

Why the usable share matters

The Ah figure on a battery applies to a long discharge (often 10 or 20 hours). A UPS drains its batteries in minutes, and at that rate a battery delivers only part of its rated capacity. The share depends on battery type, discharge time and cut off voltage: the right value is in the battery manufacturer's constant power discharge table. The tool does not suggest one.

What this tool does not cover

Inrush currents, harmonic loads, temperature, battery ageing and the UPS overload curve are not included. The result is a first estimate: for critical loads the selection must be made by a qualified engineer using the UPS and battery manufacturers' data.