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UPS power rating

How to work out the UPS power rating you need: list your devices, convert watts to VA, add up the loads and allow extra for motors and laser printers.

The apparent power (S) of a UPS is the load, in volt-amperes (VA), that it is designed to support. UPS systems are available in power ratings from 300 VA up to 5,000,000 VA (or even more).

To calculate the UPS power rating you need, follow the steps below:

  1. Make a list of the devices and equipment your UPS will protect.
  2. Write down the volts and amps (or watts) of every device.
  3. For each device, calculate:
    • VA = 1.43 x Watts (when the watts are given)
    • VA = Volts x Amps (when the amps are given)
  4. Add up the VA of all the devices.
  5. Multiply this total by 1.3, for greater protection and to leave room for adding more devices to the UPS protection in the future.

To calculate the real power of the UPS, multiply the apparent power (S, in VA) by the UPS Power Factor. See Tescom UPS systems with Power Factor = 1 here.

If your UPS will support devices with electric motors or laser printers, add more VA capacity to your requirements to account for the high inrush current drawn when these devices start up.

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