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How to choose the right sized UPS for your gear

Q: “What are the differences between Watts and VA and how are these terms used in choosing a UPS?”

A: Power consumption information on computer loads is often specified in a way that makes it difficult to correctly choose an uninterruptible power supply – a UPS. It’s important to understand the distinction between the Watt and Volt-Ampere (VA) measures for UPS load sizing. Both ratings are important; be sure to read the Word for the Day definitions of these terms.

The power in Watts is the real power drawn by the equipment – it defines the actual power purchased from the utility company and the heat loading generated by the equipment. VA is called the “apparent power” and is calculated as the product of the voltage applied to the equipment times the current drawn by the equipment. The VA rating is used for sizing wiring, circuit breakers and power protection equipment such as a UPS.

The VA and Watt ratings for some types of electrical loads, like incandescent light bulbs, are nearly identical. However, for computer equipment the Watt and VA ratings can differ significantly. The VA rating is always at least equal to or larger than the Watt rating. The ratio of the Watt-to-VA rating is called the Load Factor and is expressed either as a number (i.e. 0.6) or a percentage (i.e. 60%).

A UPS has both Watt ratings and VA ratings and neither may be exceeded. In most cases, UPS manufacturers only publish the VA rating. However, it is a standard in the industry that a typical load factor is approximately 60% of the VA rating. Therefore, it is safe to assume that the Watt rating of a UPS is 60% of the published VA rating.

Since UPS manufacturers often list only the VA rating on their nameplates, you run the risk of choosing a system that appears to be correctly sized based on VA ratings but actually exceeds the UPS Watt rating. By making sure the VA rating of your load – the combined ratings of your computer and all other equipment you intend to plug into the UPS – to be no greater than 60% of the VA rating of the UPS, you’ll never exceed the Watt rating of the UPS. Unless you’re absolutely certain 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. While this conservative sizing approach might lead you to choose an oversized UPS with a longer run time than absolutely necessary, the added protection isn’t wasted. It pays off in longer battery life and better handling of multiple power outages.

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