Three UPS topologies, one that matters for three-phase critical power. Here is how double-conversion, line-interactive and eco-mode actually differ, and how to choose.
Every UPS is one of three designs. What separates them is how much of the time your load runs through the inverter, and that decides how clean the output is and how fast the unit reacts when the power misbehaves.
Standby (offline). The load normally runs on raw utility; the inverter only starts when the supply fails. Cheap, but there is a brief transfer gap and no conditioning. You see it only in small single-phase units, never in a serious three-phase system.
Line-interactive. Adds an automatic voltage regulator that trims high and low voltage without drawing from the battery. Fine for small servers and network closets where sags and swells are the main issue. It still transfers to battery on a real outage, and in practice it is a single-phase technology, three-phase line-interactive products are rare and small.
Online double-conversion. The load runs through the rectifier and inverter continuously: incoming AC becomes DC, then is rebuilt as a clean sine wave. The battery sits on the DC bus, so there is effectively zero transfer time when utility fails, and the output is isolated from whatever the input is doing. This is the topology for three-phase critical power.
By the time a load needs three-phase input it is usually a data hall, an imaging suite, an industrial process or a building’s mechanical plant, places where a few milliseconds of bad power, or a transfer notch, is not acceptable. Double-conversion gives a continuously regenerated output, tight frequency regulation (which matters when a generator is the upstream source), and a clean static-bypass path for fault current and service. That is why essentially every three-phase system we supply, Eaton 93PM, APC Galaxy VS, the Vertiv Liebert range, is online double-conversion.
The long-standing knock on double-conversion is the energy it burns running the load through two full conversions. Eco-mode answers that (Eaton brands it Energy Saver System; others call it economy or smart-eco mode): the unit feeds the load through the static bypass while holding the inverter ready, then snaps back to full double-conversion in a couple of milliseconds if the input drifts. Manufacturers document eco-mode efficiency at or above 99%, against roughly 94 to 97% for the same unit in full online mode.
The trade-off is real. In eco-mode the load is briefly exposed to raw input during the transfer back, so it suits sites with stable utility and tolerant equipment, not the most sensitive loads. A common compromise is eco-mode off-hours and full online during the day, or eco on some modules and online on others.
Topology drives operating cost, not just purchase price. A 200 kW load at 95% versus 99% efficiency is on the order of 8 kW of difference, power you pay for once to lose as heat and again to cool out of the room. Over a ten-year service life that adds up to real money, so weigh efficiency alongside the sticker. Both sizing and redundancy feed into the same math.
For three-phase critical power the real question is rarely “double-conversion or not”, it is which double-conversion platform, how much redundancy, and whether eco-mode earns its place. Tell us the load, the upstream source (utility or generator) and how sensitive the equipment is, and we will spec it from any major brand and size it properly. Browse systems by application on the data-centre, industrial and healthcare pages, or talk it through with an engineer.
In practice, yes. Line-interactive and standby designs exist as small single-phase products; any serious three-phase system for a data centre, hospital or industrial load is online double-conversion because the load cannot tolerate transfer gaps or unconditioned power.
Eco-mode (Energy Saver System on Eaton, economy/smart-eco elsewhere) runs the load through the static bypass with the inverter held ready, lifting efficiency above 99%. It is safe for sites with stable utility and tolerant loads; for the most sensitive equipment, full online mode is the conservative choice.
Manufacturers document eco-mode at or above 99% efficiency, versus roughly 94 to 97% for the same unit in full online double-conversion. On a large load that few-percent gap is several kilowatts of continuous loss.
If the load is a few small servers on clean utility, line-interactive is enough. If it is three-phase, mission-critical, or fed from a generator, you want online double-conversion for the zero-transfer, fully regulated output.
Tell us the application and we will come back within one business day, sizing, the right system, install and a price. Three-phase installs usually need a licensed electrician, so let us know if you have one.