Online Double-Conversion UPS
An online double-conversion UPS rebuilds the power feeding your equipment continuously: incoming AC is rectified to DC, and an inverter creates a new, clean AC waveform for the load. When the utility fails, nothing switches over, because the inverter was already carrying the load. Zero transfer time and complete isolation from the input are the two properties that separate this topology from everything else.
The power path, end to end
Double conversion: the load stays on the inverter; the static bypass exists for maintenance and overload, not for everyday sags.
When double-conversion is the right call
Three situations settle the question. First, loads that cannot tolerate a transfer gap: servers mid-write, laboratory instruments, medical electronics, PLC and SCADA hardware. Second, sites with poor power quality, where a line-interactive unit would hammer its battery with constant transfers while an online unit simply absorbs the noise. Third, anything behind a standby generator: generator frequency drifts during load steps, and an online UPS rides through drift that would force other topologies onto battery again and again.
What it costs you: a few points of efficiency
The two conversion stages typically cost a few percent versus a line-interactive design running in bypass. Current platforms answer this with high-efficiency modes that pass utility power through when it is clean and return to double conversion within milliseconds when it drifts. Whether to enable such a mode depends on the load and the site; it is a commissioning decision, not a default.
Platforms we supply and service
Single phase online units we quote regularly: APC Smart-UPS On-Line, Eaton 9PX and 9155, Vertiv GXT5, Tripp Lite SmartOnline and Xtreme Power Conversion online models, from 1 kVA rack units to 20 kVA towers with external battery cabinets. Above that band, the same topology carries into three phase platforms; see three phase UPS systems. For stocked equipment with published specifications, browse UPS Plus Battery.
Installation, commissioning and service
We deliver, install and commission online UPS systems across Canada, including transfer testing under load before handover. Batteries remain the wear item: 3 to 5 years for VRLA in a conditioned room, with heat shortening that window. Preventive maintenance catches aging batteries, tired capacitors and fan wear before they become an outage; battery replacement is available for every brand above.
Compare the three topologies
VI line-interactive
VFD standby
On a sag or outage: nothing changes at the output. The inverter already carries the load, so there is no transfer gap at all.
Best fit: servers, storage, network cores, lab and medical instruments, anywhere a millisecond matters. This is the class this page covers.
Shallow sags and outages: the unit corrects shallow sags with an autotransformer; on an outage it switches to battery with a brief gap, commonly a few milliseconds on published specs.
Best fit: edge closets and equipment with forgiving power supplies, where the price difference matters more than the gap.
When the utility fails: the load runs on raw utility until the unit detects a problem, then switches to battery; published transfer specs commonly run several milliseconds.
Best fit: desktops and non-critical gear. It is the least protection of the three and does not belong in a server room.
Common questions
A line-interactive UPS passes utility power through and switches to battery when it fails, with a transfer time of a few milliseconds. An online double-conversion UPS converts AC to DC and back continuously, so the load never sees the raw utility waveform and there is no transfer time at all.
Whenever transfer time or waveform quality can hurt the load: servers and storage, laboratory and medical electronics, industrial controls, and any equipment running downstream of a generator, where frequency drift makes line-interactive units switch to battery repeatedly.
The conversion stages cost a few percent of efficiency compared to line-interactive designs. Many current models offer high-efficiency modes that pass power through under clean utility conditions and return to double conversion when the input drifts, a setting worth reviewing with your service provider rather than enabling blindly.
More UPS formats
Zero transfer gap, zero drama
Tell us the load and the room and you get a written proposal: the right double-conversion unit, batteries sized to your runtime, installation and a service plan.
