Usually seen when sizing uninterruptable power supplies the Volt-Amp is often confused with Watts, they are not the same. The power requirement of a piece of equipment is expressed in Watts of Volt Amps (VA), the most common example being of course the 100W light bulb.
A Watt is the actual or true power used or dissipated by the device
A Volt Amp is called the apparent power and is obtained by multiplying the voltage by the current drawn by an alternating current device.
This is a small but significant difference. The Watt rating is used for measuring the power used and resultant heat. The VA is used for sizing calculations.
There are two types of load, resistive and reactive.
A light bulb is resistive but a computer or other electronic device is reactive.
If we calculated a the power used for a resistive light bulb operating at 240volts and drawing 0.25ampere it is a matter of applying a simple formula;
Power = Voltage x Current or 240 x 0.25
60 watts, the VA rating would be exactly the same.
Where the load is reactive a power factor must be used. With reactive loads a certain amount of power is absorbed by and subsequently released by the device. This power amount is called the reactive power or the difference between apparent and true power.
In an example where a computing device has an impedance of 120 Ohms using Ohms law (current = voltage/resistance) 240/120 will produce a current figure of 2 amps.
Using the same formula as above to get power
Power = Voltage x Current or 240 x 2
The apparent power would then be 480VA
Since the load is electronic, a power factor must be applied. Different devices will have different power factors; in this case the computer has a power factor of 0.9
Applying the power factor to the apparent power results in a watt figure of 480 x 0.9 = 432 watts
This difference between the apparent power and true power is reactive power, in this example 48 Volt Amps.
The vast majority of modern large computer devices now have very high power factors, usually close to 1 but smaller devices such as PC’s may be as low as 0.65
However, UPS devices do not have this high power factor. UPS devices are rated in VA with a stated power factor. The power factor is generally accepted to be 0.6 for UPS devices designed to power PC’s and other small devices.
A typical 500VA UPS would deliver 300 Watts. To complicate things even further large UPS’s now have very high power factors, approaching 1
The UPS will have both maximum VA and Watt ratings that cannot be exceeded.
Careful thought needs to be applied to the correct sizing of UPS’s taking into account the nature of the load and design specifications of the UPS itself in order to avoid errors. The safest approach is to keep the load at less than 60% of the VA rating of the UPS or seek expert advice
