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Watt Ratings Differs From Volt Amp Ratings


This document helps explain the differences between Watts and VA and explains how the terms are correctly and incorrectly used in specifying power protection equipment. Many people are confused about the distinction between the Watt and Volt-Amp (V-A) measures for UPS load sizing.

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This article was created to address a common question.



The power drawn by computing equipment is expressed in Watts or Volt-Amps (VA). The power in Watts is the real power drawn by the equipment. Volt-Amps is called the "apparent power" and is the product of the voltage applied to the equipment times the current drawn by the equipment.  Both Watt and VA ratings have a use and purpose. The Watt rating determines the actual power purchased from the utility company and the heat loading generated by the equipment. The VA rating is used for sizing wiring and circuit breakers.

The VA and Watt ratings for some types of electrical loads, like incandescent light bulbs, are identical. However, for computer 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%).

The power rating of the UPS

UPS have both Watt ratings and VA ratings. 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. In recent times this has begun to change as many UPS' have begun to have closer Watt and VA output ratings.

How to avoid sizing errors

Using APC sizing guidelines or an APC Configuration can help avoid these problems, as the load power values are verified.  Equipment nameplate ratings are often in VA, which makes it difficult to know the Watt ratings. If using equipment nameplate ratings for sizing, a user might configure a system which appears to be correctly sized based on VA ratings but actually exceeds the UPS Watt rating.

By sizing the VA rating of a load to be no greater than 60% of the VA rating of the UPS, it is impossible to exceed the Watt rating of the UPS. Therefore, unless you have high certainty of the Watt ratings of the loads, the safest approach is to keep the sum of the load nameplate ratings below 60% of the UPS VA rating. Note that this conservative sizing approach will typically give rise to an oversized UPS and a larger run time than expected. If optimization of the system and an accurate run time are required, use APC sizing and Worldwide Web configuration tools.


Power consumption information on computer loads is often not specified in a way that allows simple sizing of a UPS. It is possible to configure systems that appear to be correctly sized but actually overload the UPS. By slightly over sizing the UPS compared with the nameplate ratings of the equipment, proper operation of the system is ensured. Over sizing also provides the side benefit of providing additional UPS backup time.

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