Online UPS vs offline UPS
One rebuilds the waveform continuously; the other passes the mains through and switches to battery when it fails. That single difference decides which one your equipment needs — and for a lot of loads, the cheaper answer is the correct one.
An online UPS rectifies incoming mains to DC and synthesises a fresh sine wave for the load, continuously. The load never touches the mains, so transfer time is zero because there is no transfer. An offline or line-interactive UPS passes conditioned mains straight through and switches to its inverter when the supply fails — under 8 ms on our units, but not zero.
What transfer time actually costs you
Eight milliseconds sounds like nothing, and for most equipment it is. A switch-mode power supply — which is what sits inside a PC, a server, a router or an NVR — holds its output through a gap of roughly 16 to 20 ms on its own capacitors. An 8 ms transfer passes underneath that entirely and the equipment never notices.
The trouble starts with anything that watches the supply rather than just consuming it. A CNC controller, a PLC, a medical imaging system or a lift drive may register the break, fault, and stop mid-operation. The machine has not been damaged; it has simply decided the supply is unreliable and protected itself. That is the scrapped job or the stalled car, and it is the whole argument for double conversion.
| Load | Sensible choice | Why |
|---|---|---|
| Desktops, NVR, routers, small servers | Offline | The power supply rides through 8 ms without noticing |
| Server rack or small data hall | Online | Redundant supplies and management cards are less forgiving |
| CNC and machine tools | Online | A controller reset mid-cycle scraps the job and can crash the tool |
| Medical imaging and theatre | Online | Break-free is a specification requirement, not a preference |
| Lifts and elevators | Online (lift-specific) | Regenerative load; a general-purpose inverter trips or is damaged |
| Billing counters, ATMs, back office | Offline | Tolerant loads, and the cost difference is real money |
| Plating and process rectifiers | Online | Deposit consistency depends on a stable supply |
Where offline is genuinely the right answer
It costs less to buy, less to run and less to maintain. An offline unit only inverts when it needs to, so it is not converting power twice for twenty-four hours a day, and there is no rectifier stage working continuously. On a tolerant load with a reasonable supply, specifying online is spending money to solve a problem you do not have.
We will say so. If the load list is offices, cameras and a billing counter, an online UPS is the wrong recommendation however much larger the invoice would be.
The question that settles it
Does anything on this circuit make a decision based on the supply, rather than just consuming it? If yes, online. If no, offline is very likely enough.
Where online is the only answer
Beyond sensitive loads, there is a second case that gets missed: a supply that is bad rather than absent. On the MIDC feeders at Chakan, Bhosari and Ranjangaon the common fault is not an outage but sag — a neighbouring induction motor starting and pulling the line down for a few hundred milliseconds, repeatedly, all shift.
An offline UPS responds to each of those by transferring to battery. Do that forty times a day and the bank is being cycled forty times a day, which is how a five-year bank becomes a two-year bank. An online UPS absorbs the same sag in its rectifier stage without touching the battery at all. On a poor feeder, double conversion is not about the milliseconds — it is about battery life.
Efficiency, and the honest version of it
Double conversion has historically been the less efficient topology because power is converted twice. Modern units narrow that gap considerably — ours run up to 96% — but an offline unit sitting in bypass is still doing less work. On a 100 kVA load running continuously the difference is a real electricity bill, and it belongs in the calculation alongside the purchase price.
What it does not justify is choosing offline for a load that cannot take the transfer. One scrapped CNC job usually costs more than a year of the efficiency difference.
What we build in each
- Online: 3 kVA to 200 kVA across three series, single or three-phase output, 0 ms transfer, up to 96% efficient, THDv under 3%
- Offline and line-interactive: 3 kVA to 60 kVA across three series, under 8 ms transfer, AVR boost and buck, single and three phase
- Both are built and load-bank tested at the works on Sinhagad Road before they ship
If you are unsure which side of the line your load sits on, send the equipment list rather than a kVA figure. The answer usually depends on one or two items on it.
Questions we get asked
Anything not covered here, call us — most of it is faster to answer on the phone.
Is an online UPS always better than an offline UPS?
No. It is better for sensitive loads and poor supplies, and unnecessary for tolerant ones. A billing counter, a camera system or an office of desktops rides through an 8 ms transfer without noticing, and specifying double conversion there is money spent on a problem that does not exist. Better means suited to the load, not more expensive.
What is the actual difference between offline and line-interactive?
Line-interactive adds automatic voltage regulation. It corrects moderate over and under-voltage by switching transformer taps without going to battery, where a plain offline unit would either pass the bad voltage through or transfer. Everything we build in this range is line-interactive with AVR boost and buck, which is why battery cycling stays low on a mildly unstable supply.
Will an 8 ms transfer restart my computer?
Almost certainly not. A switch-mode power supply holds its output through roughly 16 to 20 ms on its own capacitors, so an 8 ms break passes underneath it. The equipment that does object is anything monitoring the supply rather than consuming it — CNC controllers, PLCs, imaging systems and lift drives.
Can I run motors on either type?
Both, with the same caveat: a direct-on-line motor draws six to eight times its running current at start, and the UPS has to survive that surge rather than just the steady load. Tell us the motor rating and the starting method — direct-on-line, star-delta, soft starter or VFD — because that single detail changes the machine more than the kVA total does.
Which is cheaper to run over ten years?
Offline, on a good supply with a tolerant load — lower purchase price and less conversion loss. Online, on a poor supply, because it does not cycle the battery on every sag. On a feeder that dips repeatedly, replacing a battery bank early twice over ten years costs more than the efficiency difference ever saves.