How Stuff Works

Uninterruptible Power Supplies Most computers run 120 volts of AC power--the kind from most wall plugs. Any fluctuations in the power supply--such as voltage sags or blackouts--can cripple a computer. Uninterruptible power supplies are designed so that there are no power interruptions.

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Continuous UPS

Online UPS, sometimes called a true UPS, is the best type you can buy. Under normal operation, the computer is always running on battery power while the line power continually runs the battery charger. Because there is no changeover time, continuous UPS units are used to supply computers, such as servers, that require a stable source. Tracing the electricity from wall outlet to output receptacles:

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Battery Charger

Converts wall power from AC to DC, which is then used to charge the battery

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Battery

Similar to a car battery, but sealed to avoid hydrogen escaping or acid spilling. Size of battery dictates the size of the UPS unit. Most newer UPS units have user-replaceable batteries

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Inverter

Changes the battery’s stored DC electricity to AC

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Transfer Switch

Selects between battery and line power to output receptacles

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Standby UPS

Sometimes called a standby power supply (SPS) or “offline UPS.” It runs off wall power unless electricity is cut off. In usually less than 5 milliseconds, it activates the inverter and supplies electricity to the computer from the battery.

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Sources: www.howstuffworks.com; PC Guide; American Power Conversion Corp.; Belkin Components

Researched by CHRISTINE FREY/For The Times