The importance of Power Factor, Crest Factor and Surge Factor

Power Factor, Crest Factor and Surge Factor are unrelated technical terms that are seldom prominently displayed, but are important and must be considered when sizing UPS systems.

Most loads are either inductive or capacitive and do not utilise all of the current they draw. They have an apparent power rating expressed as Volt-Amps or VA reflecting their total current load and a Wattage rating defining the power they actually consume. The ratio of apparent to actual power is known as the power factor.

Information technology equipment including servers, routers, hubs and storage systems almost always uses power factor corrected power supplies. With a power factor near unity, the difference between their VA and Wattage rating - their reactive power - can be ignored. However many desktop PCs can have a power factor as low as 0.65, which means that their apparent power requirement is 50% higher than the actual power consumed.

When sizing UPS systems, therefore, it is essential to specify sufficient capacity in terms of both the wattage power and the current they can deliver. A safety margin can be ensured by choosing a UPS with a wattage rating equal to or greater than the VA rating of the load.

Crest factor is the ratio between the instantaneous peak current and the RMS or average current drawn by a load. IT equipment typically exhibits a crest factor of around 1.4, whereas a PC's could be 2 or 3. For factors above 1.4, the UPS must be able to deliver the peak current demanded by the load to avoid distortion of its supply voltage. Crest factors are a function of the source AC waveform as well as the load characteristics. Compared with a sinusoidal source, a UPS can reduce the crest factor of a load such as a PC.

The surge factor of a load is the ratio of its startup inrush current to its steady state current demand. For large disk arrays, this can be approximately 1.5, whereas for typical computer systems it is approximately 1.15. UPSs must have sufficient capacity to support the overall surge factor of their driven load.