UPS units are sold in three main designs. They differ in how the power reaches your equipment, and so in how well they protect it. Picking the right one saves money and avoids underprotecting something important.
Key takeaways
- Offline (standby) UPS is the simplest and cheapest; power passes straight through until the mains fails.
- Line-interactive adds voltage correction, so it handles brownouts and surges without using the battery.
- Online (double-conversion) UPS always supplies power from its inverter, giving the cleanest output and no break.
- The more sensitive or critical the load, the more you should move toward online.
1. Offline or standby
In normal use the load is fed directly from the mains, with basic surge filtering. When the mains fails, the UPS switches to the battery, which takes a few milliseconds. It suits simple, low-cost needs such as a home PC. It does little about low or high voltage other than switching to battery.
2. Line-interactive
A line-interactive UPS has an automatic voltage regulator (AVR) that raises or lowers the voltage within a range, so a brownout does not drain the battery. It also switches to battery on a failure, again with a short transfer time. It is a good balance for offices, shops, networks and small servers where voltage varies. A UPS such as the APC Back-UPS Pro 900VA is of this broad class.
3. Online or double-conversion
An online UPS converts the mains to DC and then back to AC all the time. The load is always on the inverter, so there is no transfer time when the mains fails, and the output voltage and frequency are clean and steady even when the input is bad. It is used for servers, medical equipment, industrial controls and data rooms. It costs more and uses a little more energy, though many have an economy mode.
4. Which to choose
- Home PC, router, TV: offline or line-interactive.
- Office, shop, small network: line-interactive or small online.
- Servers, medical, laboratory, industrial control: online.
- Three-phase plant and large data rooms: large online or modular systems; see three-phase UPS for industry.
5. Other points to compare
Compare the runtime on the battery, the output waveform (pure sine wave matters for sensitive loads), the power factor, the battery type, the noise and the cost of replacement batteries. See how to size a UPS.
Next step
Tell us what you are protecting and we will suggest the type and size; use the quotation form or see our commercial UPS solutions.
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