UPS units come in three main designs. A standby UPS passes utility power straight through and switches to battery when it fails. A line-interactive UPS adds voltage regulation so it rides through sags and swells without using the battery. An online double-conversion UPS rebuilds every cycle of power through its inverter, so the load never sees the utility directly. Each costs more and protects more than the one before.
Why the design matters
Power problems are not just outages. Equipment also sees brief sags when large motors start, swells when loads switch off, spikes from switching and lightning, frequency drift on generators, and distorted waveforms. Different UPS designs deal with these in different ways. Picking the right one means matching the design to how sensitive the equipment is and how bad the power is where it sits. Our guide to power quality and harmonics describes the problems in more detail.
Standby (offline) UPS
In a standby UPS, the load runs directly on utility power, usually through basic surge suppression and filtering. A charger keeps the battery topped up. When input voltage drops outside a set range or disappears, a transfer switch moves the load to the inverter, which runs from the battery. When power returns, it switches back.
Standby units are inexpensive, compact and efficient, because nothing is converted in normal operation. Their weaknesses are the short transfer time each time they switch, and the fact that they pass most power quality problems straight through to the load. Some smaller units produce a stepped approximation of a sine wave on battery, which some equipment does not like. They suit individual workstations, small network equipment and home offices.
Line-interactive UPS
A line-interactive UPS adds an automatic voltage regulator, usually a transformer with taps that the UPS switches between, so it can raise or lower the output voltage during sags and swells without going to battery. The inverter is connected all the time and also acts as the charger. Because the inverter is already running, transfer to battery during an outage is faster than in a standby unit.
Line-interactive units are a common middle ground for network closets, small server rooms and equipment in areas with occasional voltage swings. They save battery cycles in places where sags are common, which helps battery life. They do not correct frequency variations or heavy waveform distortion, and there is still a brief transfer when the battery takes over.
Online double-conversion UPS
An online UPS converts all incoming AC to DC with a rectifier, then rebuilds clean AC with an inverter. The battery sits on the DC link between them. When input power fails, the rectifier stops and the battery simply keeps the DC link up, so there is no transfer and no break in the output. The load is isolated from voltage, frequency and waveform problems on the input, which also makes online units the usual choice on generator power.
The cost is efficiency and heat. Converting all the power twice wastes some energy as heat, which the room's cooling then has to remove. Many larger online units offer a high-efficiency or eco mode that runs the load on bypass and switches to the inverter when a problem appears. That improves efficiency but brings back some of the exposure an online design is meant to remove, so use it only after weighing the risk for the load in question.
Online units are standard for data centres, larger server rooms, healthcare and laboratory equipment, process controls and anywhere the load must see no disturbance at all. Most three-phase UPS systems in commercial buildings are online designs; see UPS systems for buildings.
What the load sees during an outage
The difference between the three designs is clearest at the moment the utility fails. The diagram compares what reaches the load.
Choosing between them
| Design | Strengths | Limits | Typical uses |
|---|---|---|---|
| Standby | Low cost, small, efficient | Transfer gap, little conditioning | Desktops, small network devices |
| Line-interactive | Voltage regulation without battery use, good value | Brief transfer, no frequency correction | Network closets, small server rooms |
| Online double conversion | No transfer, full isolation from input problems, generator friendly | Higher cost, heat and losses | Data centres, healthcare, controls, three-phase systems |
Questions that help decide:
- How much does an interruption or reboot of this equipment cost?
- Will the UPS run on generator power, which often varies in frequency during load changes?
- How large is the load, and is it single-phase or three-phase?
- Where will the UPS sit, and can that space remove its heat?
- How will it be serviced without shutting down the load? See UPS bypass and maintenance.
A typical case
A medical clinic has a small server rack, a phone system, a few network switches in a closet on each floor and diagnostic equipment in two exam rooms. The building has a standby generator that serves life safety loads and a few selected circuits. Staff have noticed that equipment sometimes reboots when the generator is tested.
Working through it, the server rack and the phone system are the loads where a reboot costs the most and where the generator's frequency swings during load changes cause trouble. An online UPS sized for the rack, the phone system and some growth, fed from a generator-backed circuit, removes both the transfer gap and the generator sensitivity. The floor closets hold switches that recover quickly; line-interactive units there give good protection at much lower cost and heat. The diagnostic equipment is checked against the manufacturer's requirements, which in many cases already specify the type of UPS they need.
The result is a mix of designs, each matched to the load, rather than one type everywhere. The same thinking applies in an office tower, a school or an industrial site.
Single-phase and three-phase units
Small standby and line-interactive units are almost always single-phase and plug into a receptacle. Larger online units serving a room or a floor are often three-phase, hard-wired and fed from a dedicated breaker, with their own input and output panels. Once a UPS is hard-wired, it becomes part of the building's electrical system: it needs a design, a permit and installation by a licensed electrical contractor, and the bypass arrangement becomes part of the design.
Watts and VA are not the same. UPS units are rated in both volt-amperes (VA) and watts. The load must fit within both. Sizing on VA alone, or adding up nameplate ratings without checking the watt rating, is a common reason a UPS trips to bypass or runs out of battery sooner than expected.
How ehZee can help
We assess loads and power conditions and recommend the topology, size and arrangement that fits the risk, as part of our UPS system design service. Battery runtime is covered in UPS batteries and runtime.
Common questions
Is an online UPS always better?
It protects more, but it costs more, uses more energy and produces more heat. For a single desktop or a small switch, a line-interactive or standby unit is often the sensible choice.
Should we use eco mode on our online UPS?
It saves energy, but the load spends most of its time on bypass power. Some operators accept that for less critical loads; others keep critical loads on full double conversion. Discuss it with the manufacturer and weigh it against what the load can tolerate.
Why does our UPS keep switching to battery?
It may be reacting to real voltage swings, to a generator with unstable frequency, or to settings that are too tight for the site. A power quality recording at the input usually shows which.