The online-versus-line-interactive decision is often made on price, then justified with a spec sheet. That is backwards. The topology should follow from two questions: how much can the load tolerate a transfer gap, and how dirty is the input power? Once those are answered, the price comparison is meaningful.
This article explains what each topology does, where the 0 ms transfer matters, and the cases where a line-interactive unit is the better engineering choice.
What is the difference between online and line-interactive?
A line-interactive UPS normally powers the load from the mains through an AVR stage. When the input fails, it switches to its inverter in 4–6 ms. The AVR corrects voltage sags and swells without using the battery, but the load sees the mains waveform most of the time.
An online UPS converts the input to DC and back to AC continuously. The load is always supplied by the inverter, and the battery sits on the DC bus. When the mains fails, the inverter continues without a transfer. The output is a regenerated sine wave with low THD.
| Criterion | Line-interactive | Online double conversion |
|---|---|---|
| Transfer to battery | 4–6 ms | 0 ms |
| Normal output | Mains, AVR-corrected | Regenerated sine wave |
| Output THD | Depends on input | <3% linear, <7% non-linear |
| Input voltage window | Moderate | Wide, 85–300 VAC on many models |
| Battery use during sags | Switches if the AVR cannot correct | Battery only when the DC bus drops |
| Efficiency at light load | Higher | Lower |
| Cost | Lower | Higher |
| Best for | POS, CCTV, office IT | Servers, medical, telecom, industrial |
The table is a starting point. The actual performance depends on the model, the load, and the input condition.
When is 0 ms transfer actually required?
Zero-millisecond transfer matters when the load has a hold-up time shorter than the transfer. A server power supply may hold up for 10–20 ms, so a 4–6 ms transfer is often invisible. A medical device, a laboratory analyzer, or a precision instrument may have a shorter hold-up or may reset on any disturbance.
The second case is a load that is sensitive to the waveform, not just the gap. A motor or a compressor can be damaged by a simulated sine wave, even if the transfer is fast. An online UPS gives a clean sine wave and a 3:1 crest factor, which is why it is specified for industrial and medical loads.
The third case is a generator-backed site. A generator’s voltage and frequency can move during a load step. An online UPS with a wide input window rides through the step and keeps the load stable. A line-interactive unit may switch to battery, drain it, and then switch back when the generator recovers.
| Load | Hold-up / sensitivity | Topology |
|---|---|---|
| Desktop PC | 10–20 ms, tolerant | Line-interactive |
| Network switch | 10–20 ms, tolerant | Line-interactive or online |
| Server | 10–20 ms, but sensitive to waveform | Online |
| Medical analyzer | Short hold-up, sensitive | Online |
| PLC / DCS | Sensitive to transfer and waveform | Online or industrial |
| Motor / compressor | Cannot use simulated sine | Online or industrial |
If the load is a server, a medical device, or an industrial controller, the 0 ms transfer and the clean waveform are worth the extra cost. If the load is a POS terminal or a CCTV recorder, a line-interactive unit with a pure sine wave output may be sufficient.
How does the input voltage range change the choice?
A wide input range keeps the UPS on mains during sags, which preserves the battery for real outages. An online UPS with an 85–300 VAC input can ride through a deep brownout without touching the battery. A line-interactive unit with a narrower input range may switch to battery, drain it, and fail to recharge before the next event.
This is the hidden cost of a line-interactive UPS in a weak-grid market. The unit may work perfectly in the lab and fail in the field because the input spends hours below its AVR range. The battery is cycled repeatedly, and the runtime degrades.
| Input condition | Line-interactive | Online with wide input |
|---|---|---|
| 220 V ±10% | Good | Good |
| 180–200 V sustained | AVR may correct, battery may cycle | Good, stays on mains |
| 160 V sustained | May transfer to battery | Good on 85–300 V models |
| 260 V sustained | AVR may correct | Good |
| Generator frequency swing | May transfer | Wide input, better ride-through |
If the site has a sustained brownout, an AVR upstream of the UPS can also solve the problem. The AVR corrects the voltage and the UPS sees a stable input. That combination is often cheaper than a larger online UPS, but it adds a maintenance item and a floor-space requirement.
What happens to the battery in a long brownout?
A UPS is not designed to carry a long brownout. If the input stays at 160 V for two hours and the UPS transfers to battery, the battery will be exhausted in minutes. The load then drops, even though the “outage” was only a voltage sag.
An online UPS with a wide input window avoids this by staying on mains. An AVR upstream avoids it by correcting the voltage before the UPS. A line-interactive UPS with a narrow AVR range does not avoid it, which is why the input range is a more important specification than the transfer time for many weak-grid sites.
The battery is also cycled more often in a line-interactive design, which shortens its life. The cost of replacing the battery every 2–3 years can erase the initial price advantage.
Which topology fits a generator-backed site?
A generator introduces voltage and frequency variation during start-up and load steps. An online UPS with a wide input range and a high input power factor is generally more stable on a generator. It also isolates the load from the generator’s waveform.
The generator must still be sized for the UPS input current plus the battery charging load. A 10 kVA UPS at full load draws about 45 A at 220 V single-phase, plus charging current. A generator that is too small will collapse when the UPS charger and the load step coincide.
A line-interactive UPS can work on a generator if the generator is stable and the input range is adequate. For a critical site, the online UPS is the safer choice, especially if the generator is shared with motor loads.
How do I compare the total cost?
Compare the capital cost, the battery replacement cost, the efficiency, and the downtime risk. A line-interactive UPS is cheaper upfront, but it may cycle the battery more often and may not protect a sensitive load. An online UPS costs more and uses a few percent more energy, but it avoids the transfer event and the battery cycling.
For a small POS or CCTV load, the line-interactive unit is usually the right lifecycle choice. For a server room, a medical device, or a production line, the online UPS is usually cheaper over the life of the project because it prevents the failures that cost more than the UPS.
| Cost element | Line-interactive | Online |
|---|---|---|
| Capital cost | Lower | Higher |
| Battery life | Shorter in a weak grid | Longer if the input is stable |
| Energy use | Lower at light load | Higher, but stable |
| Load risk | Transfer gap, waveform | Low |
| Maintenance | Battery, AVR | Battery, fans, filter |
The decision should be documented. If the buyer chooses line-interactive, the quotation should state the input window and the loads that are excluded. If the buyer chooses online, the quotation should state the battery runtime and the input range.
The decision in one page
- Use line-interactive for small commercial loads that tolerate a 4–6 ms transfer and a simulated sine wave.
- Use online for servers, medical devices, telecom, and industrial control.
- Check the input window against the measured site voltage, not the nominal value.
- On a generator, size the generator for the UPS input current and charging load.
- Compare battery life and downtime risk, not just the purchase price.
The topology is the first decision; the capacity and the battery come after. A 10 kVA line-interactive UPS and a 10 kVA online UPS are not interchangeable, even though the nameplate looks the same.
