When choosing an uninterruptible power supply (UPS), you’ll see two power values – watts and VA (volt-amperes). What’s the difference between them?

Some Basic Electricity

The two most common units you deal with in electricity are the voltage and the current. The voltage (measured in volts) can be thought of as the “pressure” or magnitude of the electric potential. The current (measured in amps) is the amount of electricity that is flowing. To use the analogy of a water pipe – volatage is the water pressure and current is the amout of water flowing.

In a direct current DC circuit (where the voltage is a constant), the amount of power used can be found by multiplying the volatage and current (or V x A – hense Volt-Amperes or VA). For example: if a flashlight uses 0.1 amps at 3 volts, the total power is 0.1 x 3 = 0.3 watts. With DC, VA and watts are always the same.

AC/DC

… is not just a rock band.

The electricity coming in from the power company does not have a constant voltage. in fact, the voltage takes the form of a sine wave as shown here.

When the power factor is 1, the voltage and current are in phase

Normally as the voltage increases the current increases as well. In this case the power can still be calculated by multiplying the voltage by the current. VA and Watts are again the same.

But some devices have what is called a capacitive input. In these cases the current is out of phase with the voltage as shown here:

If the voltage and current are out of phase, the power factor is less than one

Since the peak current and voltage don’t happen at the same time, the actual power delivered will be less than the peak voltage multiplied by the peak current. How much less is called the power factor.

You can’t always know how much power a device actually needs. When sizing a UPS you should look at the devices you plan to plug in. If they specify the wattage of the device – great. If they specify the current, mulitply it by the input voltage (usually 110 volts) to get the VA. It’s safest to assume this is also the wattage for electronic devices, even though the amount of power a device actually uses is sometimes less. The problem is – most of your devices won’t specify the power factor.

Choose a UPS that provides a wattage adequate to power the load you are going to plug in. Remember – only devices with hard drives really need a battery backup – the rest will be fine on a surge protector.

See also: Uninterruptible Power Supplies Vs. Surge Protectors – Which do you need?