Online UPS Selection Guide
An online UPS has a rectifier, a DC busbar and an inverter in series; when choosing, you need to know the normal, battery and bypass operating modes and the VFI class.
This guide explains what to look at when choosing products in the Online UPS category.
Operating modes of the double-conversion design
In an online UPS, the rectifier, the DC busbar to which the battery bank is connected, and the inverter are in series. The unit operates in three modes:
- Normal operation: The energy taken from the mains is rectified; this energy charges the battery and feeds the inverter. The load is fed from the inverter.
- Battery operation: When the mains fails or the input goes out of limits, the rectifier stops and the inverter takes energy from the battery. No switching takes place on the load side.
- Bypass operation: In case of overload or inverter failure, the static bypass transfers the load directly to the mains; during this time the load is exposed to disturbances on the mains.
In the IEC 62040-3 classification, double-conversion operation is indicated by the VFI code: the output voltage and frequency are independent of variations on the mains. Because the energy is converted twice, the losses of this operating mode are higher than the others, and the loss is released to the environment as heat.
Which model for which phase configuration
The expressions 1f/1f, 3f/1f and 3f/3f in product names give the number of input and output phases. In Siel names, the Sld 11 models are listed as 1f/1f, the Sld 31 models as 3f/1f, and the SLDS 33 models as 3f/3f.
- 1f/1f: Loads such as servers, network and measurement devices fed from a single-phase line.
- 3f/1f: Installations where all loads are single-phase but the power must be taken from a three-phase mains.
- 3f/3f: Three-phase loads and large power distributed across the phases at the output.
Power, battery and connection
For online UPS units, active power (kW) is given alongside apparent power (kVA); the ratio of the two is the unit's output power factor. In ABB records the power factor appears as 0.9 or 1; for this reason the kW that two units with the same kVA value can carry may differ. Also make the comparison with your load's kW value.
On the battery side, the "with battery" and "without battery" options of the Siel SLDS 33 models are separate products. In ABB records it is stated whether the battery is a replaceable battery or a battery module; the battery runtime at full load and the number of external battery modules that can be connected are also stated for many models.
The mounting style appears in ABB records as tower (Tower) or rack-tower (Rack-Tower); the Siel Srt 11 models are also rack-tower type. A rack-tower unit can be placed upright on the floor or mounted horizontally inside a cabinet; for power distribution inside the cabinet, see the PDU power distribution units page. ABB records also give the connection type (IEC C14, IEC C20 or hardwired) and the number of C13 and C19 outputs; a hardwired unit is connected to the installation by cable, not by plug.
Pre-installation checklist
- Load list: kVA and kW totals, loads with high inrush current.
- Supply: number of input phases, neutral and earthing, and whether the bypass input will be fed separately.
- Protection and cable: the protective device and cross-section of the input, bypass and output lines. For a preliminary calculation, the current and fuse calculation from active power and cable current carrying capacity calculation tools can be used.
- Battery: the required runtime, battery placement and ambient temperature.
- Space and heat: weight, service clearance and ventilation. In some of the ABB records, power loss and heat dissipation are given in W; ventilation is planned according to this value.
Exact values are determined according to the manufacturer's installation manual and the project.
Brands in this category
- Siel: 1f/1f Sld 11, 3f/1f Sld 31, rack-tower Srt 11 and 3f/3f SLDS 33 models; the SLDS 33 models are listed with an LCD touch screen in the product name.
- ABB: Double-conversion models in tower and rack-tower form, such as PowerValue 11T G2 and 11RT G2. Because the product name contains only the manufacturer order code, look up the power rating in the technical specifications.
- Schneider Electric: 230 V SPM online models.
Ordering notes
Products are sold by the piece. Schneider Electric Easy UPS SRV online models, three-phase Easy UPS units, and parts such as battery modules, battery cabinets and network cards are on the parent category page, UPS uninterruptible power supplies; for low-power units with AVR, see the line interactive UPS page. You can reach ABB products by typing the manufacturer code (for example 4NWP100150R0001) into the search box.
Comparison and selection table
| Selection Criterion | 1–3 kVA Single-Phase (1/1) Online UPS | 6–10 kVA (1/1 or 3/1 Phase) Online UPS | 10–200+ kVA Three-Phase (3/3) Online UPS |
|---|---|---|---|
| Phase Configuration | 1P In – 1P Out (230 V AC, socket or terminal block) | 1P/1P or 3P In – 1P Out (hardwired terminal block) | 3P In – 3P Out (400 V AC + Neutral) |
| Topology | True double-conversion (VFI), 0 ms transfer, pure sine wave | Double-conversion with static & maintenance bypass | Double-conversion IGBT, low THDi, parallel N+1 ready |
| Form Factor & Battery | Tower or 19" Rackmount, internal sealed batteries | Tower or Rack, internal batteries + external battery cabinets | Floor-standing cabinet with scalable external battery banks |
| Typical Application | Servers, PLC racks, telecom closets, security systems | IT rooms, medical clinics, laboratory analyzers | Data centers, industrial process lines, hospital critical buses |
Frequently asked questions
What is eco mode in an online UPS?
In eco mode, the load is fed directly from the mains through the bypass line as long as the mains is suitable; because double conversion is bypassed, efficiency rises. When the mains goes outside tolerance, the unit returns to double-conversion operation. In this mode the output depends on the mains (VFD in the IEC 62040-3 classification) and the UPS does not condition the mains. In ABB records, the efficiencies in normal operation and in eco mode are given separately; whether the mode is used is decided according to the sensitivity of the load.
What is the difference between static bypass and maintenance bypass?
The static bypass is inside the unit and works electronically: in case of overload or inverter failure it transfers the load to the mains automatically. The maintenance bypass is operated manually; when the unit is taken in for service or replaced, it feeds the load directly from the mains, outside the UPS electronics. If the load must stay energized during maintenance as well, a maintenance bypass is added to the project. In some of the ABB records, external maintenance bypass information is included in the technical specifications.
What is the difference between an online UPS with battery and one without battery?
A product with battery is a unit listed together with its batteries. A product without battery covers only the UPS; the battery bank is selected separately according to the required runtime. A unit without battery is preferred for installations that require a long runtime or where the batteries will be placed in a separate space. The voltage, capacity and connection method of the battery bank are determined according to the UPS manufacturer's data.
How do I choose between a 3f/1f and a 3f/3f online UPS?
The loads are decisive. If all loads are single-phase and can be distributed from a single output line, a 3f/1f design can be used; in normal operation the current drawn from the mains is divided across the three phases. In bypass operation, however, the single-phase load is fed from a single phase of the mains; the bypass line and neutral are sized according to the manufacturer data. If there are three-phase loads, or the total power is too large for a single-phase output, a 3f/3f design is chosen and single-phase loads are distributed evenly across the phases.
What is the difference between a tower and a rack-tower online UPS?
A tower unit is placed upright on the floor or on a shelf. A rack-tower unit can be used either this way or mounted horizontally inside a cabinet. If it will be installed in a cabinet, the unit's depth, weight and the height it will occupy in the cabinet are checked in the technical specifications; if a battery module will be added, space is reserved in the cabinet for it as well.