PC Hardware in a Nutshell, Second Edition

BPS Characteristics

Here are the most important characteristics of a BPS:

Volt-Ampere (VA) rating

The VA rating of a BPS specifies the maximum power the UPS can supply, and is determined by the capacity of the inverter. VA rating is the product of nominal AC output voltage and the maximum amperage rating of the inverter. For example, Barbara’s 120V APC Back-UPS Pro 650 can supply about 5.4A (650VA/120V). Connecting a load greater than the amperage rating of the inverter overloads the inverter and soon destroys it unless the BPS has current-limiting circuitry. Watts equal VA only for 100% resistive loads (e.g., a light bulb). If the load includes capacitive or inductive components, as do PC power supplies, the draw in VA is equal to Wattage divided by the Power Factor (PF) of the load. Most PC power supplies have Power Factors of 0.65 to 0.7. For example, Robert’s APC Smart-UPS 1000 is rated at 1000VA but only 670 Watts, which means that APC assumes a PF of 0.67 when rating wattage for this unit.

Run time

The run time of a BPS is determined by many factors, including battery type and condition, Amp-hour capacity, and state of charge; ambient temperature; inverter efficiency; and percentage load. Of those, percentage load is most variable. The number of Amp-hours a battery can supply depends on how many amps you draw from it, which means the relationship between load and run time is not linear. For example, our APC Back-UPS 600 can supply 600VA for five minutes, but can supply 300VA ...