"UPS" is a single name for three completely different architectures: a unit that only enters the circuit when the power fails, a unit that corrects unsuitable grid voltage, and a unit that always feeds the load from its own inverter. This page explains the three topologies from an engineering perspective, introduces the standard classification framework, and shows how the right choice is made for each load. For a broader overview, the page Industrial UPS is the starting point.
The three main UPS architectures
The fundamental difference comes down to one question: where is the load fed from in normal operation? In offline and line-interactive units, the load is on mains and the inverter stands by; in online units, the load is fed from the inverter output under all conditions. This single difference determines everything else: whether there is a transfer time, how much input disturbance is filtered, and the voltage swing the unit tolerates without going to battery.
Offline (Standby) UPS
The simplest architecture in terms of circuit structure. The main path connects the grid directly to the load, and when the monitoring circuit detects a voltage sag or loss it activates the transfer switch so the load is handed to the battery inverter. So there is a short but real interruption on transfer, and in normal operation any grid distortion or fluctuation reaches the load as is. This architecture makes sense for non-critical loads, not process, telecom or laboratory equipment.
Line-interactive UPS
Here an autotransformer with switchable taps — usually called an AVR — is added to the path. When grid voltage sags or rises, instead of going to battery the unit changes the transformer tap and brings the output voltage back into the acceptable range; the result is preserved battery life. But two limitations remain: on a complete outage the transfer to the inverter still occurs, and the output frequency still follows the grid.
Online double-conversion UPS
The incoming power is first rectified, then the inverter builds a clean AC output from the DC bus. The load is never connected directly to the grid; the battery sits on that same DC bus, and a power outage only changes the source feeding the bus, not the load path. That is why the transfer time on an outage is zero and the output is regulated in amplitude and frequency independently of the grid. A bypass path is provided for overload or servicing, and the transfer to it, unlike an outage, does take time.
Online vs. line-interactive UPS in practice
This question comes up a lot when buying, because the difference between the two does not always show on the spec sheet. It should be viewed along four axes:
- Load supply path: In line-interactive, the load is normally on the grid; in online, it is always on the inverter.
- Transfer time: In line-interactive, the transfer to battery has a real interruption; in online it is zero, because no transfer occurs in the load path.
- Disturbance isolation: Line-interactive corrects the voltage but passes the input waveform and frequency through; online rebuilds the output from scratch.
- The price of this advantage: In online units all the power passes through two conversion stages, so continuous losses and heat are higher and thermal design becomes seriously important.
| Criterion | Offline | Line-interactive | Online (double conversion) |
|---|---|---|---|
| Load supply in normal operation | Directly from the grid | Grid, with voltage correction | from the unit's inverter |
| Transfer time during a power outage | Noticeable interruption | has an interruption | Zero |
| Output voltage regulation | None | Stepped, with tap changer | continuously, by the inverter |
| Output frequency regulation | Follows the grid | Follows the grid | Off-grid |
| Grid disturbance isolation | Negligible | Minor | Full on the normal path |
| Continuous losses and heat | Low | Low | Higher; requires ventilation |
| Suitable load | Non-critical | Semi-sensitive | Critical and sensitive |
Classification framework based on IEC 62040-3
In industrial purchasing, adjectives like "true online" are not evidence on their own. The international reference for stating UPS performance is the IEC 62040-3 with the general title "Method of specifying the performance and test requirements". The logic of this standard is that, instead of a qualitative description, the unit's performance is declared with a classification code; the familiar codes VFD, VI and VFI are defined within this same framework, and the exact meaning of each code and its test conditions can only be cited from the text of the standard itself. The text and tables of this standard are copyrighted and are not reproduced here; what matters in technical correspondence is that the vendor writes its declared classification code and the reference clause number of its test, not a marketing adjective. Two other members of this family are also used in technical documents: IEC 62040-1 for safety and IEC 62040-2 for electromagnetic compatibility.
Specifications of a real online unit
So the topology discussion does not stay abstract, the table below gives the specifications of Keyhan Energy's online double-conversion UPS, taken from the official product catalog:
| Parameter | Value |
|---|---|
| Capacity | 1000 VA / 800 W |
| Topology | Online, double conversion |
| Output waveform | Pure sine wave |
| Output voltage | 220 V, ±2% to ±3% |
| Output frequency | 50 Hz ±1% |
| Input voltage | 170 to 265 V, single-phase |
| Transfer time | 0 ms (4 ms to bypass) |
| Efficiency | Up to 90% on mains, up to 85% on battery |
| Battery | 12 V sealed lead-acid or 12.8 V lithium iron phosphate |
| Charge current | 15 A for SLA and 20 A for lithium, up to 100 Ah |
| Operating temperature | −10 to +50 °C |
| Noise | Less than 50 dB |
| Protection | 16 hardware and software layers, alarm and LCD display |
| Control software | In-house; designed and written by the company itself |
The technical point of this table is the last row: the inverter regulation logic, charge management and protection layers were designed and written in-house and are in our own hands. To assess the feasibility of any adaptation to a specific application, send the load description and installation conditions via the Contact us page. Related product lines: Telecom Rectifier and Cathodic Protection Switching Power Supply.
Which topology for which load?
- Critical, interruption-sensitive load: Servers, telecom equipment, instrumentation and process loads whose reset is costly. Here online is the default choice, and the discussion is about capacity and backup duration, not topology.
- Semi-sensitive load on a weak grid: Where the main problem is voltage sags and swells rather than frequent outages; line-interactive can be sufficient, provided the load's tolerance of the transfer interruption has been checked.
- Non-critical load: an offline unit is sufficient and the extra cost is not justified.
The second decision is as important as the first: backup time is set by the battery bank, not the topology, and the choice between Lead-acid and Lithium has a direct effect on life, weight and charging method; the options are on the Industrial Battery page.
Frequently asked questions
Does "zero milliseconds" mean there is no switching at all?
meaning that when input power fails, no transfer takes place in the load path; the load is fed from the same inverter before and after the outage. Transfer to bypass is a separate event and takes time; for our product it is stated as 4 ms.
If an AVR also stabilizes the voltage, what is the difference between line-interactive and online?
An AVR corrects the voltage in steps by switching transformer taps, and passes the input waveform and frequency straight through. The online architecture rebuilds the output from scratch, so regulation is continuous and the output frequency is stabilized independently of the grid.
Why is an online UPS less efficient?
Because all the power passes through two conversion stages, each with its own losses, whereas in a line-interactive unit the power normally takes the direct grid path. These losses become heat and must be accounted for in the ventilation of the installation site.
How can I tell whether a seller is really selling an online unit?
Ask for the declared classification code based on IEC 62040-3 and which clause of the test requirements it is based on. If the answer is only marketing adjectives, there is no technical evidence.
How do I determine the required capacity?
Start from the list of actual loads, the power and power factor of each, and the required backup time; then select a matching battery bank. To check your own case, use our Technical services or send your load list via the Contact us page.
Next step
A full description of the application and acceptance tests is on the Industrial UPS page. For loads that must stay on through long outages, combining the UPS with local power generation is considered; Solar Inverter, Solar Power Plant and the Solar system calculation tool are the starting point; our construction track record includes the 400 kW Bardaskan plant, the 400 kW ISACO plant and the 200 kW Loshan plant. For ancillary items, the Equipment Supply page is also available. The product warranty is provided according to the company's stated terms.
Contact: +98 996 617 1347 · info@keyhanenergy.net