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

To choose the UPS you need, you should consider the following parameters:

1. Create a list of devices that the UPS will support.
2. Calculate the Volts and Amps (or Watts) of all devices.
3. For each device, consider:
   • VA = 1.43 x Watts (if you know the Watts)
     • VA = Volts x Amps (if you know the Amps)
4. Estimate the total VA of the devices.
5. Multiply this figure by 1.3, for larger capacity and to have a margin of battery capacity for the UPS.

To calculate the actual UPS requirements, you should multiply the apparent power (S in VA) by the Power Factor of the UPS. See Tescom's UPS with Power Factor = 1.

If the UPS will support devices that have electric motors or laser printers, estimate higher VA requirements to support the inrush current of these devices.