Neil Rasmussen, American Power
Many people are confused about the distinction between watt (W) and volt-ampere (VA) measures for uninterruptible power supply (UPS) load sizing. Some manufacturers of UPS equipment have added to this confusion by failing to distinguish between these measures?and in some cases, have erroneously equated the two.
The volt-ampere rating system is better for matching the load to the UPS because the fundamental factor that limits the output capacity of a UPS is its output current, which is more closely related to volt-amperes than to watts.
These two alternating-current (AC) power measurements are related as follows:
Watts = Volt-amperes x Power Factor =
Volts x Amperes x Power Factor
Volts = 120 or 230V, typical
Amperes = load current
Power factor = between 0 and 1
Power factor is a number between 0 and 1 that represents the fraction of the load current that provides useful energy (or watts) to the load. For most electrical devices, other than electric heaters and incandescent light bulbs, some current flows out of the load without delivering watts, causing the volt-ampere rating to be larger than the load watt rating.
Virtually all modern computers, for example, use power supplies with capacitor-input switching, which exhibits a power factor of 0.6 to 0.7. Because UPS systems are volt-
ampere-limited devices, and computer loads vary from 0.6 for PCs to 0.7 for minicomputers and larger devices, the UPS watt rating for computer-type loads must be 60% to 70% of the UPS volt-ampere rating.
For example, American Power Conversion measured a typical 386 desktop PC with monitor, hard drive, tape backup, mouse and Ethernet card to find its total power consumption (in watts), load current (in amperes) and volt-ampere requirement. For a 120V AC system, it found total wattage to be 230W, total amps to be 3.04A, AC voltage to be 120V AC, volt-amperes to be 365VA and power factor to be 0.63. Similar power factors are obtained for other computer configurations and 230V systems.
When a manufacturer provides a watt rating for a UPS, without a separate power factor or volt-ampere rating, the user must assume that the rating applies for a power factor of 1, which means volt-amperes equal watts. In such a case, a computer-type load will be 60% to 70% of the published watt rating. When a UPS system is rated in volt-amperes, the volt-ampere rating for computer loads is equal to the published rating, and the watt rating is between 60% and 70% of the published volt-ampere rating. Some manufacturers of UPSs rate their systems in both volt-amperes and watts. In this case, power consumption for sizing is specified in volt-amperes, but can be converted to watts by multiplying by approximately 0.65 for computer equipment.
Neil Rasmussen is vice president of engineering at American Power Conversion?s research and design facility in Billerica, MA.