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Understanding The Power Rating Of Uninterruptible Power Supply (UPS)

   


Many people are confused about the distinction between the Watt and Volt-Amp (VA) measures. Many manufacturers of UPS equipment add to this confusion by failing to distinguish between these measures, in some cases they erroneously equate Watts and VA.

The power drawn by computing equipment is expressed in Watts or Volt-Amperes (VA).

Both Watt and VA ratings have a use and purpose. The Watt rating determines the actual power purchased from the electricity department and the heat loading generated by the equipment. The VA rating is used for sizing wiring and circuit breakers.

In computers equipment, the Watt and VA ratings can differ significantly, with the VA rating always being equal to or larger than the Watt rating. The ratio of the Watt to VA rating is called the “Power Factor” and is expressed either as a number (i.e. 0.7) or a percentage (i.e. 70%).

Power Factor = Watt / VA


The Watt rating for a computer is typically 60 to 70 percent of its VA rating. This means that the Watts drawn by a typical computer are 60% to 70% of the VA rating.

UPS have both Watt ratings and VA ratings. When using an UPS neither the Watt nor the VA rating of a UPS may be exceeded.

In most cases, UPS manufacturers only publish the VA rating of the UPS. However, it is a standard in the industry that the Watt rating is approximately 60% of the VA rating, this being the typical power factor of common loads. Therefore, it is safe to assume that the Watt rating of the UPS is 60% of the published VA rating.

Let us see some examples where the difference in VA and Watt could create some problem.

Consider the case of a 600VA UPS. The user wants to power a 500W heater with this UPS. The heater has a Watt rating of 500W and a VA rating of 500VA with a power factor of 1. Although the VA rating of the load is 500VA, which is within the VA rating of the UPS, the UPS will not power this load. That is because the 500W rating of the load exceeds the Watt rating of the UPS, which is 60% of 600VA, which is around 360W. Therefore, unless you have high certainty of the Watt ratings of the loads, the safest approach is to keep the sum of the load ratings below 60% of the UPS VA rating.
     

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