All about VA and Watts

VA is a simple abbreviation for Volts time Amperes.
Volt is the tension and ampere is the amount of current that passes thanks to the difference of voltage (tension) between two points.

Ratings of UPS are done in VA because UPS are powered by batteries.

Watts = VA times PF (power factor - alos called cos phi).

The power factor is the phase difference between voltage and current. It mainly depends on the kind of equipment connected.

Each UPS has a VA rating and most of the manufacturers also give you a watts rating, assuming a power factor of generally 0.6. This assumption is usually correct, but some equipment might even have a smaller PF.

Dimensionning an UPS will be done follwing the criteria:

- Maximum power the UPS can deliver: in watts. Compare the power (in watts) to the one of the equipment connected. The UPS must be able to deliver at least 5% more than what you connect. The same calculation can be done using the VA, assuming you know the real current consumption of your equipment (not the max or the fuse rating, which are way above the real consumption). You can for example refer to the manufacturer specs in normal operation or even cut a power chord and insert a ammeter (connected in serial).
In case you measure the current drained by your equipment, you are fully on the safe side since the PF does not enter in the equiation anymore. I recommend this last method, you just need to sacrifice one power chord.

- The amount of time during which you expect the connected equipment to run powered by UPS only. This calculation can vary from manufacturer to manufacturer and trying to calculate it manually can lead to mistakes. Manufacturer websites often include automatic calculations for their equipment. See for example http://www.apcc.com/template/size/apc/

Most manufacturers use a VA rating based on 5 minutes and not an hour, which is VERY misleading. For example a 500 VA UPS can support a 500 VA load during 5 minutes, or a 250 VA load during 10 minutes.
Don't try to calculate this yourself and always check with the manufacturer as runtimes vary even within the same brand !

Laser printers cannot be connected to UPS because they drain a very high current when they start up (to warm up the paper) and the UPS would be overloaded.
However, most UPS include now special plugs for printers that are protected against overvoltage, but that are not UPS backed up.