When the lights go out

If there’s a power failure and your PC is on, you could lose data. Windows could blue-screen every time you try to boot, especially if you’re running RAID 5 and the power cuts out while you’re writing data to your hard drives. The resulting power surge that occurs when the power comes back on could fry your motherboard, processor, and power supply, as well.

Considering the small fortune you’ve spent building it, why would you be willing to put your PC at risk? Three little letters could save you a whole heap of trouble: U-P-S. An uninterruptible power supply can mean the difference between tears and wondering what everyone else is complaining about.

A typical UPS
A typical UPS.

A UPS does exactly what the name implies: it provides uninterrupted power. There are many different types of uninterruptible power supplies, ranging in size from ones that can fit in a 3.5″ hard drive bay to massive mechanical devices that weigh many tonnes.

If you walk into your nearest PC shop, you’ll most likely have the option to choose from an offline (or standby) UPS and a line interactive UPS.

An offline UPS is very basic, providing only surge protection and battery backup, and typically supplies 5-20 minutes of standby power. They usually won’t include a battery monitoring tool or self diagnostic capability. These limitations make them hard to recommend unless you are really on a shoe-string budget.

A line interactive UPS is similar to an offline UPS, but it can also compensate for under and over voltage situations thanks to a variable-voltage autotransformer. An autotransformer is an electrical transformer that can add or subtract powered coils of wire to control the output voltage. Autotransformers can compensate for a wide range of voltages, but due to cost and complexity concerns they are only designed to cover 190 V to 240 V for 220 V power. Anything higher or lower would result in the battery being switched on. If you have to measure the voltage coming out of your wall socket you’ll see that the voltage rarely ever stays at 220 V.

A beeeeg, industrial UPS
A beeeeg, industrial UPS.

Let’s say that your PC at load consumes 440 watts. That works out to 2 amps at 220 V. If the voltage dropped to 200 V the UPS would draw 2.2 amps to compensate.

So how do you decide what size UPS is right for you? If you have a decent power supply with active PFC, a good rule of thumb is 1:1. So, if you have a 500 watt PSU, 500 KvA should be sufficient. If you have a rather old, or no name brand, power supply with a passive PFC the ratio moves up to 1:1.5. This means a 300 watt power supply would need a 450 KvA UPS. Getting a larger UPS won’t damage your PC, and it will allow you to keep your PC on for longer in the case of a power failure.

A decent UPS is much better than a standard surge protector.