Guides
There are three main UPS topologies: standby, line-interactive and double-conversion (online), plus multi-mode designs that switch between efficiency and full conditioning. They differ in how much of the utility's disturbance reaches the load, how often they use the battery, and how efficient they are.
A standby UPS lets the load run straight from utility power and switches to battery only when it detects a problem. Eaton's UPS basics paper describes it as typically the lowest-cost type, with limited conditioning in some designs, and notes that on battery it may produce a stepped or modified sine wave rather than a true sine wave. That suits small desktop loads; larger servers and storage with more complex power supplies may not run properly on it. It has no place in a data-centre power train.
A line-interactive UPS regulates the utility voltage up or down before passing it to the load, and uses the battery to guard against frequency problems. It is common for small server and network loads where the utility is reasonably clean. Schneider Electric's White Paper 79 notes that above 5000 VA line-interactive has historically been impractical because of size and cost, which is why three-phase data-centre UPS are double conversion.
A double-conversion UPS converts all incoming power twice, all the time: the rectifier turns AC into DC and the inverter rebuilds clean AC for the load. Eaton's paper explains that this isolates critical loads from raw utility power; if the input falls outside its limits the rectifier shuts off and the inverter carries on from the battery, and on a severe overload or a rectifier or inverter failure the static switch moves the load to the bypass source. It is the least efficient topology but the standard choice for mission-critical systems, and Eaton names it the preferred topology in electrically poor environments such as industrial sites. How each stage works is explained in how a data-centre UPS works.
A multi-mode UPS runs in a high-efficiency mode while the utility is clean and returns to double conversion when it is not. Makers implement this differently, and the difference matters. Schneider Electric's Galaxy VS manual describes two modes: in ECO mode the load runs on the static bypass and, depending on the transfer conditions, may see an interruption of up to 10 ms when the UPS returns to double conversion; in eConversion the inverter keeps running in parallel and the transfer back to double conversion is uninterrupted. Read the manual for your model before enabling either.
| Standby | Line-interactive | Double conversion | |
|---|---|---|---|
| Normal power path | Utility, unconditioned | Utility, voltage-regulated | Rectifier and inverter |
| Voltage disturbances | Battery | Regulated, then battery | Absorbed by the conversion |
| Frequency disturbances | Battery | Battery | Absorbed by the conversion |
| Output on battery | May be stepped or modified sine | Depends on design | Sine wave |
| Typical use | Desktop and point of sale | Small servers and network gear | Data centres, critical and industrial loads |
Less than is often claimed, for IT loads. Schneider Electric's White Paper 79 sets out the IEC 62040-3 output tolerance curve, under which the output voltage of a UPS may be zero for up to 10 milliseconds, and calls the idea that mission-critical equipment needs zero transfer time a myth; it shows a computer power supply holding its output for 18 milliseconds after its input was removed. Other loads, such as motors and some industrial controls, behave differently, which is one reason double conversion is preferred for them.
Double conversion. Eaton's generator guidance notes that standby and line-interactive UPS use the battery to keep frequency variations from the load, while a double-conversion UPS rebuilds the sine wave and filters them in normal operation, preserving the battery while the generator runs. More in UPS and generator compatibility. For three-phase models by maker and topology, see the three-phase UPS reference by manufacturer; for service on any of them, UPS maintenance and repair for data centres.
Double conversion (online), sometimes with a high-efficiency mode the operator can enable. It isolates the load from utility disturbances and is the standard for mission-critical loads.
No. A line-interactive UPS regulates utility voltage and passes it through; an online (double-conversion) UPS rebuilds the output from DC all the time.
A mode in which the load runs on the static bypass while the utility is within tolerance, to save energy. On the Galaxy VS a transfer back to double conversion may interrupt the load for up to 10 ms, so it is a site decision.
Schneider Electric's White Paper 79 calls that a myth: under IEC 62040-3 the output may be zero for up to 10 milliseconds, and computer power supplies hold up longer than that.
Sources, checked 2026-10-04:
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