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

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

1. Create a list of equipment devices that will protect your UPS.
2. Record the Volts and Amps (or Watts) of all devices.
3. For each device, calculate:
   • VA = 1.43 x Watts (when Watts are given)
     • VA= Volts x Amps (when Amps are given)
4. Add all VA's devices together.
5. Multiply this sum by 1.3, for greater protection and to allow for future addition of devices to the UPS protection.

To calculate the actual power of the UPS you need to multiply the apparent power (S in VA) by the Power Factor of the UPS. See here Tescom UPS with Power Factor = 1.

If your UPS will support devices that have electric motors or laser printers, add more VA capacity to your requirements to account for the high power input that occurs when these devices start up.