Here’s a useful article that backs up some of our other posts on sizing for Uninterruptible Power Supplies: Uninterruptible Power Supply: Consider Required Watts Before Buying a UPS
Legacy computer systems had a rectified input power supply that takes current in surges, rather than in a smooth sine-wave fashion. The results of this were that the power factor (the ratio of apparent power to true power) of computer power supplies worked out to be 0.7, that is for each 100VA of apparent power, the UPS needed to deliver 70Watts of true power. This is why UPS have traditionally had two ratings – VA and Watts and typically these tended to be different by a factor of, yes that’s right 0.7.
These power surges caused by computer power supplies can play havoc with the utility supply which is why standards have been introduced to make computer power supplies more utility friendly, and they do this by incorporating circuits to have what is called power factor correction, raising the power factor from the traditional 0.7 to a level approaching 1.
The effect of this on Uninterruptible Power Supply Sizing is clear. On your legacy computers you could add up the VA ratings and your UPS would be practically guaranteed to be sized correctly. However, systems with modern compliant power supplies are different, and you need to make sure you don’t overload the WATTS rating of the UPS.
For example, 4x250VA legacy systems could safely be powered from a 1000VA/700W UPS. Now, you would need to ensure that the 4x250VA systems were powered by a UPS System rated at at least 1000W – about 1500VA (for a traditionally rated UPS). The Eaton 9130 range of Online Double Conversion UPS Systems go some way to overcoming this dilemma by having their systems rated at an impressive 0.9pf, which means that for every 1000VA, the UPS can supply 900W.
If your power supply doesn’t say or if you are unsure, the safe bet is to take the VA rating you have and multiply it by 1.4, in this instance your UPS will in the majority of cases be appropriately rated.
