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Appliance Science: The uninterruptible power supply

Where will you be when the lights go out? Still working on your computer? Yes, if you have a UPS. Appliance Science looks at how Uninterruptible Power Supplies (UPSes) work.

In a perfect world, your power would never go out, or have surges that could stop your appliances from working. We don't live in a perfect world, though: we live in a world of power glitches, of brownouts and downed power lines. For some appliances, these are more than just annoyances: an unexpected power outage can corrupt the data on your computer or send your home security system into a panic. And medical devices like CPAPs can end up choking you if the power unexpectedly stops. So, what can you do to protect your devices from these problems? Install an uninterruptible power supply, or UPS.

The job of the UPS is to provide continuous power to these critical devices, smoothing out the problems and providing a backup power source when the lights go out. They do this by putting a backup power source between your device and the power socket, then constantly monitoring the flow of power coming from the wall socket and out to your device.

There are different types of UPS, but those designed for computers and small appliances are usually in-line types, where the incoming power is converted from alternating current (AC) to direct current (DC), then fed into a battery. At the same time, the reverse is happening, with some of this current being converted back into AC current, which is then fed out to the device. If the power flowing into the device is interrupted, the battery takes up the slack, keeping the current flowing. The device being powered never knows that there has been a power outage.

Colin McDonald/CNET

Well, it never knows for as long as the battery lasts. A UPS can only keep a device powered as long as there is energy stored in the onboard battery. That's the Achilles' heel of these devices, and one of the major questions you need to consider when buying one. The bigger the battery, the longer the UPS can keep up this flow of power. But the bigger the battery, the bigger the UPS has to be to hold it.

This amount of energy stored in a UPS is measured in something called volt amps (VA), which represent how much electrical power it can deliver over time. This determines how long devices connected to the UPS will last. The VA rating will often be accompanied by a watt rating, which indicates how much electrical energy the UPS can deliver at any one moment, which determines how many devices can be powered by the UPS.

The APC ES 550G, a small UPS that offers 550VA of power storage.

Although it is notoriously hard to come up with a definitive figure for how long a UPS will keep your device going, many of the manufacturers offer basic UPS calculators. These allow you to input the number and type of devices you want to have a battery back up, and suggest which products might be appropriate. They are available from manufacturers such as APC and Tripp Lite.

How long a UPS will last depends on how much you connect to it, and how efficiently these connected devices use this power. Some devices suck the power down in a way that is inefficient for the UPS to keep up with, so they won't last as long. This is called the power factor. Why this happens is rather complex, but as a general rule, devices that use motors have a low power factor, so you shouldn't run appliances like dishwashers and washing machines off a small UPS. The same is true of devices such as laser printers, which draw a large amount of power suddenly when they start (called inrush current). That's no problem for the power from your wall socket, but a UPS can't handle that, and some could be damaged by this sudden power surge.

Most UPSes will also include a USB port and software. When running on a PC connected to the UPS, this monitors the level of charge left, and when it runs low, shuts the computer down. This clean shutdown will save your files and protect your data.

If you want to keep more than a single computer running, you'll need a bigger battery. Whole House UPS systems are built into your power system, usually between the circuit panel that holds your fuses and the electricity meter. When the power goes out, they step in and keep the power flow going in the entire house. There are several different types here, such as line interactive and delta conversion, which can handle the larger load of the multiple devices you'll find in the modern home. These are a significant investment, though, so they really are only needed if you live in an area that has frequent power outages.


The $85 Cyberpower CP685AVR, like many UPSes, offers multiple power sockets, some of which are battery-backed up, and others that only offer surge protection.