What a UPS is
and when you need one

A surge-protected extension lead only acts when a voltage spike arrives, for example during a storm, and diverts it so it doesn't reach your equipment. If the power goes, even for a second, the extension lead can do nothing: the equipment switches off abruptly, just as if it were plugged into the wall. A UPS has a battery and, when it detects that the power has gone, it starts supplying the equipment from it, so a brief dropout goes unnoticed and a long cut gives you time to shut down cleanly. Many UPSs also include surge protection, so in practice they do what the extension lead does and more. What you shouldn't do is assume that an expensive extension lead protects against power cuts: it protects against something else. And on many UPSs not every outlet is battery-backed; some are surge-only, and you need to check which before plugging in.

They're two ways of expressing power. VA, volt-amperes, is apparent power; W, watts, is the real power the equipment draws. Watts are always equal to or lower than VA, because they're obtained by multiplying the VA by the UPS's power factor, and that factor is less than one. That's why a UPS advertised with a big VA figure can deliver quite a few fewer watts than it seems. The number that matters when choosing is W: add up the real consumption of what you'll connect, which is on each device's label or datasheet, and choose a UPS whose wattage exceeds it with headroom. If the manufacturer only gives VA and not watts, be wary or ask for them. And remember that power tells you how much it can carry, not for how long: that's runtime, which is a different figure.

It depends on the battery and, above all, on how much the connected equipment draws: the same UPS lasts far longer with a router and a NAS than with a desktop computer and a large monitor. Manufacturers publish runtime tables or calculators by load, and that's the right way to find out, because a single figure on the box is usually measured at a light load. For most offices and homes the runtime needn't be long: it has to be enough to ride out brief dropouts, which are the most common, and to shut down the NAS or server cleanly in a long cut. If you need to keep working for hours without power, an office UPS isn't the tool: that's another conversation, with another budget. Bear in mind too that batteries wear out over the years and real runtime drops, so it's worth testing it every now and then, not just on the day you buy it.

It depends on what you're protecting and what the mains supply is like where you are. An offline, or standby, UPS passes mains power straight through and only switches to battery when there's a cut; it's the simplest and is usually enough for a home computer or a router. A line-interactive UPS also corrects voltage rises and dips without using the battery, and is the most sensible choice for most small offices, NAS units and modest servers. An online, or double-conversion, UPS always generates the power from its own electronics, so the equipment never sees the mains; it's what goes in front of critical servers or in places with a very unstable supply. The European standard IEC 62040-3 classifies them as VFD, VI and VFI, in case you see that on the datasheet. More protection means more cost, more heat and more noise, so you don't need online for everything.

It's the feature that turns a UPS into more than a battery. The UPS connects over USB, or over the network on models that support it, to the equipment it protects, and tells it that the power has gone and how much battery is left. With that, the NAS, server or computer can shut itself down properly, closing files and databases, before the battery runs out. Without that connection, the UPS holds on for a few minutes and then the equipment switches off abruptly just as it would without a UPS, only a little later; if the cut lasts longer than the battery, you've gained nothing. Many NAS units accept a USB UPS directly from their settings, and some can signal other devices on the network to shut down too. It's the first thing to configure after plugging it in, and the thing most often left undone.

Anything that draws a lot of power in one go and doesn't store data. The laser printer is the classic example: when its fuser heats up it suddenly demands a lot of power and can overload the UPS or drain the battery in no time. Heaters, fan heaters, coffee machines or any appliance that heats up make no sense either, for the same reason. What should be connected is whatever stores data or can't stop: the NAS, the server, the computer running the management software, the router and the switch, so the network and connection stay up, and the camera recorder if there is one. Monitors are optional: connecting them lets you see what's happening during the cut, but they use battery you may need for a clean shutdown. Before plugging in, check which of the UPS's outlets are battery-backed and which only offer surge protection, because many models have both kinds.