UPS basics: how an uninterruptible supply works

What does an uninterruptible power system do?

A UPS sits between the wall outlet and the devices you want to protect. Under normal conditions it passes mains power through and keeps its battery charged. If the supply fails, it switches to the battery in a few milliseconds, which is quick enough that most electronics never notice. Many also filter surges and correct small voltage swings.

The core purpose is a graceful shutdown or a short bridge. A desktop can save files and power down, a router stays online through a flicker, and a generator has time to start and take over. For longer outage cover at household scale, see home battery backup.

What types of UPS exist?

Three designs cover almost every product.

• Standby (offline). The simplest and cheapest. Equipment runs from mains, and the unit switches to battery when it detects a failure. Suits home computers and routers.
• Line-interactive. Adds a transformer that corrects over- and under-voltage without using the battery, which helps where the grid is unstable. A common choice for home offices and small servers.
• Online (double conversion). The equipment always runs from the inverter, with mains feeding the battery in the background, so there is no switching delay. More expensive and used for critical loads.

Pick the least expensive type that protects what you actually need.

How do you size a UPS?

Ratings come in volt-amperes (VA) and watts, and the watt figure is the one that limits real loads. Add the watt draw of everything you will plug in, then leave a margin of at least 20 to 30 percent. The device labels or power supplies list the wattage.

Runtime follows from the battery's energy. As a rough illustration, a small UPS with a 100 watt-hour battery feeding a 50 W router and modem would last in the order of an hour or less after allowing for losses, and the same unit with a 300 W desktop would last minutes. Stated runtimes assume a fixed load, and a battery loses capacity with age, so plan to replace it every few years. For terms such as capacity and power, see solar battery terms explained.

What should and should not go on a UPS?

Good candidates are computers, routers, modems, network storage and a small television or lights for a short cut. Poor candidates are anything that heats or has a large motor: space heaters, kettles, microwaves, power tools and fridges. Laser printers draw a large surge and are usually left off. The manufacturer's manual lists the equipment it is meant to support, and some medical devices need a unit rated for that use, so check with the device maker.

Keep the UPS in a ventilated spot away from heat, and in winter avoid leaving it in an unheated space where cold shortens battery life. The cold effects on batteries are covered in the cold weather guide.

How does a UPS compare with other backup options?

A UPS is a bridge. A portable power station, a generator or a home battery provides longer cover, and some home batteries and hybrid inverters offer the same near-instant switchover for the circuits they protect. The overview in backup power during outages puts the choices side by side, and energy storage types for homes shows where batteries sit among other storage forms.

Solar systems also have a UPS-like question: whether the panels will keep working in a blackout. They normally will not unless designed for it, as explained in solar panels in a power outage. A buyer who needs both quick switchover and long runtime can use a small UPS for the sensitive electronics alongside a larger backup source.

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