What is the difference between the “VA rating” and “W rating” of an Inverter/UPS?

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What is the difference between the “VA rating” and “W rating” of an Inverter/UPS?

The capacity of an Inverter/UPS is specified in terms of its VA rating and W (Watt) rating. The Watt rating relates to the amount of power it can deliver, and the VA rating relates to the amount of current it can deliver. Neither the VA nor the W rating of an Inverter/UPS can be exceeded.

The Watt rating of a load represents the “actual power” consumed by it while the VA rating is the “apparent power” consumed by it and is larger than the actual power due to some currents called reactive or harmonic currents that flow in and out of loads without actually delivering any power to it.  The ratio of the actual power to the VA rating is also called the “Power Factor” (PF). For many types of electrical equipment the difference between apparent power and actual power is very slight and can actually be ignored, but for some computers the difference is very large and important. In a study done by PC magazine, it was found that typical personal computer systems exhibit a power factor of 0.65 which means that the apparent power (VA) was 50% larger than the actual power (Watts). Normally most Inverter/UPS can handle loads with power factor ranging from 0.7 to 1.

The best approach to size an Inverter/UPS is to use the Watt rating of the load and ensure that the Watt rating of the Inverter/UPS is greater than the VA rating of all the loads put together. This ensures that even if you don’t know the power factor of the individual loads, the Inverter/UPS capacity has the requisite safety margin built into it.

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