Most UPS articles stop at how the two types work. That part is covered in Choosing the Right UPS Systems for Your Critical Infrastructure: a line-interactive UPS conditions voltage with automatic voltage regulation (AVR) and switches to battery when the supply fails, while an online double-conversion UPS runs your load through its inverter all the time.
This article is for the next question. Once you know the difference, what does choosing one over the other do to your running costs, your battery replacement cycle and your exposure to downtime? It uses the figures NPS already publishes for the Eaton, APC and Socomec ranges. For sizing, runtime and redundancy, read The Complete UPS Buyer’s Guide for Australian Businesses.
Online vs Line-Interactive: The Practical Differences
| Line-interactive | Online double-conversion | |
|---|---|---|
| Efficiency on the NPS product pages | Eaton 5PX: up to 99% | Eaton 9PX: up to 95% in online mode, 98% in high-efficiency mode. APC Smart-UPS SRT: 98 percent in green mode |
| When the mains is poor | AVR corrects sags and surges without switching to battery | Equipment is isolated from voltage fluctuations, frequency variations and harmonics |
| When the supply fails | Switches to battery | Zero transfer time |
| NPS examples | Eaton 5PX, Eaton 5P, Socomec Netys PR | Eaton 9PX, APC Smart-UPS SRT, Socomec Netys RT2, Socomec Itys |
Running Cost: Read the Efficiency Figure Properly
Efficiency is the usual objection to online UPS systems, so it is worth comparing the published figures rather than assuming. The NPS product pages tie efficiency directly to running cost: the Eaton 5PX is described as reducing cooling and utility costs, and the APC Smart-UPS SRT as saving utility and cooling costs without compromising performance or reliability.
When you compare datasheets, check three things:
- The efficiency figure for the mode you actually plan to run. The Eaton 9PX, for example, is rated differently in online mode and in high-efficiency mode.
- How the UPS protects your load while it is in that mode.
- The room the UPS will sit in. Battery capacity is rated at an ambient temperature of 20°C or 25°C, so a hot or poorly ventilated room works against both batteries and running cost. Read why UPS systems and dust don’t mix, and how computer room cooling protects power equipment.
Battery Life: Fewer Battery Events, Longer Battery Life
Batteries are the part of a UPS most likely to fail, and the part least likely to warn you before they do. How often a UPS goes onto battery therefore matters, and both types have features aimed at it:
- The Socomec Netys PR uses AVR to stabilise the output voltage and avoid switching to battery mode, which saves the battery for real power cuts.
- The Socomec Netys RT2 and Itys have a wide input voltage tolerance, which limits the number of switchovers to battery mode and significantly prolongs battery life.
The battery technology then sets the replacement cycle. NPS guidance is that VRLA batteries typically last 3 to 5 years and lithium-ion batteries 8 to 10 years. VRLA costs less up front but needs more frequent replacement and can take 3 to 6 hours to reach 90% capacity after a discharge. Lithium-ion costs more up front, is up to 45% lighter, and gives a lower total cost of ownership through fewer replacements. The VRLA vs lithium-ion guide compares them in full.
Whichever you choose, plan battery testing and replacement from day one, and know the signs your batteries need replacing. NPS provides battery maintenance and replacement as a dedicated service.
Your Mains Supply Matters More Than the Brochure
The right type depends on what is happening on your supply, and many sites have never measured it. A UPS that frequently switches to battery is one of the signs that a power quality audit is worth doing. The audit logs supply voltage, sags, swells, transients and harmonics over a representative period, typically around a week, and gives you a written report on what was found.
That data turns the choice from a guess into a decision. A supply with occasional sags and brownouts is the kind of problem AVR is designed to correct. A supply with harmonics or frequency variations is the kind of problem an online UPS isolates your equipment from. Equipment that cannot tolerate fluctuations or outages makes the answer clearer. Hope Media, a Sydney broadcaster, relies on continuous, clean power for its transmission and studio equipment, so NPS supplied an Eaton 93T 15kVA online double-conversion UPS, commissioned in August 2026. It filters power quality events before they reach the equipment, and an extended outage gives the team runtime to move to generator power or shut down safely. Read what a power quality audit finds for more examples.
Generator, Surge Protection and the Rest of the Chain
Neither type works alone. A generator is a companion to a UPS, not a replacement: the UPS carries the load instantly when the supply fails and the generator sustains it for as long as it has fuel. If a generator is in the picture, check the UPS suits generator supply. The Socomec Itys, for example, regulates output voltage and frequency constantly, which makes it compatible with generator sets.
A UPS is also not rated to absorb the energy of a lightning-induced transient, so surge protective devices belong upstream. Read Lightning Protection Systems and Surge Protection for how the two fit together. Then make sure the UPS is installed and checked properly: NPS handles installation and commissioning and UPS preventative maintenance.
Five Questions to Put to Any UPS Supplier
- What is the efficiency in the mode I will run, and how is my load protected in that mode?
- How often will this unit switch to battery on my supply, and what input voltage window does it accept?
- Which battery technology does it use, how long should the batteries last in my room, and who replaces them?
- Is it suited to generator supply?
- What do installation, commissioning and servicing include?
Choosing the Right UPS With NPS
NPS is an authorised partner of Eaton, APC by Schneider Electric, Socomec and Vertiv, and supplies line-interactive, online, modular and industrial UPS systems. NPS is vendor agnostic, so it recommends the right UPS for your environment and load requirements, not the easiest sale. For over 30 years NPS has supplied and serviced UPS systems across Australia and New Zealand, with offices in Sydney, Melbourne, Brisbane and Perth. Browse the line-interactive, online and industrial UPS ranges.
Frequently Asked Questions About Online and Line-Interactive UPS
Is an online UPS more expensive to run than a line-interactive UPS?
Not necessarily. Efficiency depends on the model and the mode it runs in. The Eaton 9PX is rated up to 95% in online mode and 98% in high-efficiency mode, and the line-interactive Eaton 5PX is rated up to 99%. Compare the datasheet figure for the mode you plan to run.
Does a UPS that often switches to battery wear its batteries faster?
NPS product information notes that a wide input voltage tolerance limits the number of switchovers to battery mode, which prolongs battery life, and that AVR avoids switching to battery for sags and surges. A UPS that frequently switches to battery is also a sign that a power quality audit is worth doing.
How long do UPS batteries last?
NPS guidance is that VRLA batteries typically last 3 to 5 years and lithium-ion batteries typically last 8 to 10 years. Temperature, usage and maintenance all affect this, so regular battery testing is important.
How do I know which type my supply needs?
Measure it. A power quality audit logs supply voltage, sags, swells, transients and harmonics over a representative period, typically around a week, and gives you a written report on what was found and what to do about it.
Do I still need surge protection with an online or line-interactive UPS?
Yes. A UPS is not rated to absorb the energy of a lightning-induced transient. Surge protective devices installed upstream clamp the transient before it reaches the UPS, and the UPS then carries the load if the supply fails.
Can I use an online UPS with a generator?
Yes. A generator takes time to start, so the UPS carries the load instantly and the generator sustains it. Critical sites use both, and NPS can confirm the UPS you choose suits generator supply.
Can NPS service a UPS I already own?
Yes. NPS maintenance is vendor agnostic, covering most UPS systems regardless of manufacturer, including APC by Schneider Electric, Eaton, Socomec and Vertiv.
Talk to NPS About Your UPS
If you are weighing online against line-interactive for a specific site, NPS can look at your equipment, your supply and your risk tolerance and recommend the right UPS. Explore the full UPS systems range, or contact the team to talk through your load.