Three designs, one standard
The international standard for UPS performance, IEC 62040-3, sorts uninterruptible power supplies into three classes by how much the output depends on the input:
| IEC 62040-3 class | Common name | What the output follows |
|---|---|---|
| VFD – voltage and frequency dependent | Offline / standby | Both mains voltage and frequency |
| VI – voltage independent | Line-interactive | Mains frequency; voltage is corrected in steps |
| VFI – voltage and frequency independent | Online double conversion | Neither; the inverter builds the output |
The class matters more than the marketing name. When two quotes describe a product differently, look for the class on the datasheet.
Offline (standby) UPS
In normal operation the load is fed straight from mains through a filter or surge suppressor. The inverter is idle and the charger keeps the battery topped up. When the input voltage leaves its window or disappears, a relay switches the load to the inverter.
- Transfer time: a short break, typically a few milliseconds up to around 10 ms, while the relay moves and the inverter starts.
- Regulation: none. The load sees whatever the grid delivers until the UPS decides it is out of range.
- Output waveform on battery: often a stepped or simulated sine wave on low-cost units.
- Where it fits: single desktop PCs, small home electronics, point-of-sale terminals on a stable grid.
Most modern computer power supplies ride through a break of this length, which is why standby units work for desktop use. Sensitive electronics, contactors and some drives may not.
Line-interactive UPS
A line-interactive unit adds an automatic voltage regulator, usually a tapped autotransformer, between mains and load. When the voltage sags or swells, the unit changes tap to bring it back into range without using the battery. The inverter is still idle in normal operation and takes over only when mains fails or moves too far.
- Transfer time: a few milliseconds, similar to standby.
- Regulation: voltage corrected in steps; frequency is passed through unchanged.
- Battery use: lower than standby on a grid with frequent sags, because the regulator handles many events without the battery.
- Where it fits: small offices, network closets and edge racks on a reasonable grid.
Line-interactive designs are efficient because most of the time power simply passes through. Their weak points are frequency disturbances, fast transients and generator supplies.
Online double-conversion UPS
Here every watt goes through two conversions. The rectifier turns mains into DC, the DC bus charges the battery, and the inverter builds a new sine wave for the load. When mains fails the inverter keeps running from the battery, so the load sees no change: transfer time is 0 ms.
- Transfer time: 0 ms from mains to battery.
- Regulation: output voltage and frequency are set by the inverter, independent of the input.
- Input window: wide; the rectifier accepts large voltage and frequency variations before the battery is called on.
- Static bypass: if the inverter is overloaded or faulted, the load moves to raw mains so it stays powered.
- Where it fits: servers and data rooms, telecom, PLCs and process control, medical and laboratory equipment, CCTV head-ends, generator-fed sites and any site with a poor grid.
The cost of this protection is efficiency and price. The rectifier and inverter are always working, so some energy turns into heat, and the unit is more complex than a standby design.
Side-by-side comparison
| Offline / standby | Line-interactive | Online double conversion | |
|---|---|---|---|
| IEC 62040-3 class | VFD | VI | VFI |
| Mains-to-battery transfer | A few ms | A few ms | 0 ms |
| Voltage regulation | None | Stepped (AVR) | Continuous |
| Frequency regulation | None | None | Yes |
| Protection from spikes and noise | Basic filter | Basic filter | Load isolated from input by the conversion |
| Behaviour on a generator | Often switches to battery | Often switches to battery | Usually stable |
| Efficiency | Highest | High | Lower in online mode; ECO mode available |
| Typical sizes | Below 2 kVA | Up to a few kVA | 1 kVA to several hundred kVA |
| Price | Lowest | Moderate | Highest |
Why industrial and IT loads use online designs
Three things push professional sites towards double conversion.
Unstable grids. Where voltage swings widely or the frequency wanders, a standby or line-interactive unit keeps transferring to battery. Each transfer is a small break and each discharge shortens battery life. An online unit absorbs the variation in its rectifier.
Generators. A diesel set’s voltage and frequency move when loads switch on and off. Online units tolerate this and stay synchronised where possible; their output is held steady either way.
Sensitive or critical loads. PLCs, drives’ control electronics, network switches and storage arrays can reset on a millisecond break or a distorted waveform. With 0 ms transfer and a clean sine output, an online unit removes that risk.
Above a few kVA, and for anything three-phase, almost all commercial products are online designs anyway.
About ECO mode
Many online UPS models offer an ECO or high-efficiency mode. In ECO mode the load runs on the static bypass and the inverter stands ready; when the input goes out of tolerance the unit transfers to the inverter. This raises efficiency considerably. On our three-phase high-frequency range, ECO mode efficiency is at least 98.5%.
The trade-off is that, while ECO mode is active, the unit behaves more like a line-interactive design: mains disturbances reach the load and there is a short transfer. Use it where the grid is clean and the load tolerates it. Keep full double conversion for critical loads or poor supplies.
Choosing in practice
- One PC or router, stable grid: a small line-interactive unit is enough.
- Network rack or small server room: online double conversion, tower or rack-mount.
- Industrial controls, labs, medical, CCTV systems: online double conversion.
- Motors, weak grids or isolation requirements: an online transformer-based industrial unit (see our transformer-based vs transformerless guide).
We make online double-conversion units only. Our high-frequency H11, H31 and H33 series cover 1–600 kVA with 0 ms mains-to-battery transfer, and the single-phase models have output power factor 1, so their kVA rating equals their kW rating. The high-frequency online range is CE tested to EN IEC 62040-1 (LVD) and EMC. For harsh industrial loads, the X11, X31 and X33 transformer-based series cover 1–800 kVA with an output isolation transformer.
Related products
- Single-phase online UPS, 1–10 kVA
- Three-phase online UPS
- Rack-mount online UPS
- Industrial transformer-based UPS
Tell us the load and the supply conditions on the contact page and we will recommend a model and rating.