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The most important decision when choosing a UPS is its type. There are two main types on the market: line-interactive and online. Both protect against power cuts, but they work differently.
How a line-interactive UPS works
Normally the load runs from the mains while the UPS watches the voltage and uses automatic voltage regulation (AVR) to correct small drops and rises without switching to battery. During an outage the UPS transfers to battery, which takes a few milliseconds (typically 2–10 ms) – unnoticeable for most computers and network devices.
- + Affordable
- + High efficiency, less heat and noise
- − Short transfer time
- − Some models output an approximated sine wave on battery
- − Cannot correct frequency or heavy electrical noise
How an online (double-conversion) UPS works
An online UPS first converts mains AC to DC and then uses an inverter to create a new, clean sine wave. The load always runs from the inverter, so there is no transfer at all during an outage (0 ms), and voltage and frequency stay stable whatever the mains does.
- + Pure sine wave with stable voltage and frequency
- + Zero transfer time
- + Works with very unstable mains and with generators
- + Most models accept external battery packs
- − More expensive
- − Lower efficiency (≈90–95%, higher in ECO mode), more heat and fan noise
Comparison table
| Line-interactive | Online | |
|---|---|---|
| Transfer time | 2–10 ms | 0 ms |
| Waveform on battery | Depends on model | Pure sine wave |
| Voltage regulation | Stepped (AVR) | Full |
| Frequency regulation | No | Yes |
| Works with generators | Limited | Well |
| Efficiency | High | Medium (high in ECO mode) |
| Price | Lower | Higher |
Which one to choose
Line-interactive is enough for office and home computers, routers, video recorders and point-of-sale systems where the mains is reasonably stable.
Choose online for servers and network racks, medical or laboratory equipment, heating boilers and pumps, places where voltage often drops or spikes, systems that run together with a generator, or when you need hours of backup with external batteries.
For large sites see high-power and modular UPS systems. To pick the right size read how to choose a UPS, and work out runtime with the calculator.