The choice between online double-conversion and line-interactive UPS is settled by one question: how much does a single broken transfer cost your load. The honest answer comes from scoring your site on four axes, load sensitivity, supply quality, generator presence and the price of an hour of downtime, then matching the score to a topology rather than to a budget headline. Buyers who lead with sticker price usually discover the real cost later, during the transfer their cheaper unit was never built to survive.
Stop asking which topology is cheaper and start asking which failure the site can tolerate. A line-interactive unit rides through small sags on a tapped regulator and transfers to battery in two to six milliseconds when mains leaves its window; an online unit never leaves the inverter, so its transfer is zero and its output is independent of the input. The difference is invisible on a clean grid and decisive the moment the load or the supply misbehaves. Score the site before you read a price list.
Programmable logic controllers, medical imaging, laboratory instruments and modern switch-mode supplies with short internal hold-up ride through only a few milliseconds. A line-interactive transfer of two to six milliseconds can drop them, because their own capacitance is already nearly spent. General-purpose servers, workstations and network gear tolerate the same transfer without a flicker. The more precision electronics in the rack, the more the score points to online, because zero transfer is the only number that leaves them unharmed.
On a stable urban grid with few neighbours and no large motors, line-interactive is hard to beat, running at 97 to 98 percent efficiency with less heat and a smaller footprint. On a weak rural feed, a site shared with welders or lift motors, or any place with frequent sags, the stepped correction of a line-interactive unit is working constantly while an online unit simply absorbs the event. Count the voltage events your site logs per month; more than a handful shifts the score toward online.
This axis alone often decides it. A generator's frequency wanders during load steps, and a topology that follows the input will pass that wander to the load or reject the source and drain its battery. An online unit accepts a wide input frequency window and holds the output steady through the whole start and stabilise sequence. Any site with a generator in the power path should specify online by default, because the alternative spends its battery arguing with the generator instead of protecting the load.
Put a number on the cost of one hour offline for this load. A ten-dollar-an-hour back office and a ten-thousand-dollar-an-hour production line score the same question completely differently. Once the hourly downtime cost clears the premium for online topology, the decision is made, because that premium is paid once and the first avoided outage repays it. Buyers who skip this number are comparing two prices without comparing the two risks, which is the root of most under-specification.
Map the scenario to the pick. Home office on a clean grid with no generator and tolerant gear: line-interactive. Small retail lane with a chiller and card terminals: online, for zero-transfer through sags. Clinic with imaging or lab instruments: online, full stop. Factory cell on a weak feeder or with a generator: online. Edge closet below ten kVA on a stable urban supply with general IT: line-interactive is acceptable. The table is a shortcut for the four-axis score, not a replacement for it, and the model-by-model comparison lives at https://www.upsboss.com/products/.
Online topology costs more to buy and runs two to four points behind line-interactive in full double conversion, which on a 100 kW load is a few kilowatts of extra loss plus the cooling to remove it. On a small load that is trivial; on a large one it deserves a line in the cost model. But against a genuinely critical load that efficiency gap is smaller than the cost of a single unplanned outage, so it is a tie-breaker between two acceptable topologies, never a reason to under-specify protection. The questions that separate the two on your site are answered in our guidance at https://www.upsboss.com/faq/.
Send us your load list, supply quality and whether a generator is in the path, and our engineers will return a scored recommendation with the right topology and a written efficiency curve. Compare the current models at https://www.upsboss.com/products/ or open a requirement through the contact page.
Key takeaway: pick the topology by scoring load sensitivity, supply quality, generator presence and downtime cost, not by price, because online double-conversion wins wherever a broken transfer is expensive and line-interactive wins only where the grid is clean, the load tolerant and no generator sits in the path.
Can a line-interactive unit ever protect sensitive equipment?
Only where the supply is clean and the equipment has real internal hold-up, which rules out most PLCs, imaging and lab gear. For those loads the two-to-six-millisecond transfer is the risk, and online topology removes it.
Is the efficiency gap between the two always worth worrying about?
On a small load it is a few watts of heat and not worth a downtime risk. On a large continuous load it is kilowatts plus cooling, so it belongs in the cost model, but still as a tie-breaker between two acceptable topologies.
What if a generator is only occasional?
Occasional still counts. The moment the generator runs, the supply wanders and the line-interactive unit either passes that wander or drains its battery. If a generator is in the path at any time, specify online so the transfer to and from it is seamless.
Contact: Frank Zhang
Phone: +86-135 5688 8641
Email: frank@upsboss.com
Add: Jufeng Road, Guangming Street, Guangming District, Shenzhen City, Guangdong Province, China
