UPS Uninterruptible Power Supplies Selection Guide
A UPS is an uninterruptible power supply that keeps powering connected devices from its battery when mains power is cut or disturbed. VFI/VI classes and selection.
What is a UPS? A UPS (uninterruptible power supply) is a device that keeps powering connected equipment from its battery when mains power is cut or disturbed. It prevents computers, servers, automation and security systems from shutting down at the moment of an outage and prevents data loss; it supplies the load for a period that depends on battery capacity. In online (double-conversion) models the load is continuously fed through the inverter, so there is no transfer time when switching to an outage.
This guide explains what to look at when choosing products in the UPS Uninterruptible Power Supplies category.
UPS types and IEC classes
| Criterion | Online (Double Conversion — VFI) UPS | Line Interactive (VI) UPS |
|---|---|---|
| Operating Principle | Continuously converts AC→DC→AC, isolating the load from grid disturbances | Feeds via AVR voltage regulation under normal grid; switches to inverter on outage |
| Transfer Time | 0 ms (zero transfer time) | 2 – 8 ms |
| Output Waveform | Pure sine wave | Stepped approximation or pure sine wave |
| Typical Application | Server rooms, PLC/automation panels, medical equipment, generator-backed sites | Workstations, routers, CCTV/NVR and POS terminals |
| Category Link | Online UPS | Line Interactive UPS |
The IEC 62040-3 standard divides UPSs into three classes according to how far the output depends on the mains voltage and frequency during normal operation; manufacturers describe their devices with this code.
- VFI (voltage and frequency independent): The output is independent of voltage and frequency variations in the mains. The double-conversion online design belongs to this class. Siel and ABB models, together with Schneider Electric SPM models, are in the online UPS subcategory; Schneider Electric Easy UPS SRV online models are listed directly in this category.
- VI (voltage independent): The output voltage is kept within certain limits, while the output frequency follows the mains. The line interactive design corresponds to this class. Siel models are in the line interactive UPS subcategory; Schneider Electric Easy UPS BVS and BVG models with AVR in the product name are listed directly in this category.
- VFD (voltage and frequency dependent): The output varies with the mains voltage and frequency. The offline, also called standby, design belongs to this class; the load is normally fed directly from the mains, and the inverter only starts up during an outage.
Voltage regulators are a separate product group for loads that do not need to run through an outage and are affected only by voltage variation.
How to read power and phase configuration
A UPS has two separate power limits: apparent power (VA or kVA) and active power (W or kW). The total of the connected loads must not exceed either value; if the selection is made by looking only at the VA value, the active power limit can be exceeded. In some Schneider Electric records, rated apparent power and rated active power are given separately in the technical specifications; in some product names both values are written together (for example 2200VA/1320W).
The phase configuration is written in the product name as input/output: 1f/1f, 3f/1f and 3f/3f in Siel names, and 3:1 and 3:3 in Schneider Electric names. A 3f/1f or 3:1 device is fed from a three-phase mains and delivers a single-phase output.
You can calculate the input and output current, the protective device and the cable cross-section for a preliminary check with the current and fuse calculation from active power and voltage drop calculation tools; exact values are determined according to manufacturer data and the project.
Battery runtime and battery options
Runtime depends on the battery type, its capacity and the load; it is read from the manufacturer's load-runtime table for your own load value.
The terms "with battery" and "without battery" in product names indicate separate products; for a device without battery, the battery bank is planned separately. Models with "Extended Runtime" in Schneider Electric Easy UPS SRV names are also separate products; check the battery layout of these models in the manufacturer data. The category also lists battery modules for Easy UPS series and empty cabinets for additional batteries; the series the module is compatible with is stated in the product name or in the technical specifications.
Accessories and installation points to check
- Series-specific accessories: Network cards, parallel kits, battery breaker boxes, parallel maintenance bypass panels and temperature sensor kits for external batteries are listed for Schneider Electric Easy UPS series. The compatible series, and for some products the power range, is stated in the product name.
- Placement and environment: Dimensions, weight and operating ambient temperature are given in the technical specifications in some Schneider Electric records; for high-power devices, the floor load capacity, service clearance and transport route are checked in advance.
- Distribution inside the cabinet: PDU power distribution units are used to distribute the UPS output to devices in a rack cabinet.
Brands in this category
- Schneider Electric: single-phase Easy UPS SRV online models, 230 V BVS and BVG models with AVR, three-phase Easy UPS 3S, 3M and 3L devices and the accessories of these series.
- Siel: online UPSs with the model names Sld, Srt and SLDS, and line interactive models; three-phase SLDS 33 models are listed with battery and without battery options.
- ABB: double-conversion online UPSs. The manufacturer order code appears in the product name; power and mounting style are in the technical specifications.
Order notes
Products are sold by the piece. Some are listed with a lead time; for these products, the delivery time is confirmed before ordering. You can reach a product directly by typing the manufacturer code (for example SRV3KI-E or 4NWP100150R0001) into the search box. Options with and without battery, or 3:1 and 3:3 options, at the same power are separate products; check the product name before ordering.
Frequently asked questions
When choosing a UPS, should I look at the VA value or the W value?
Both. The VA value is limited by the current the device can carry, and the W value by the active power it can deliver, and the load must not exceed either limit. With low power factor loads the VA limit is reached first, and with high power factor loads the W limit is reached first. For this reason, the W and VA totals are calculated separately in the load list and compared with the UPS's two values.
What does AVR do in a line interactive UPS?
AVR stands for automatic voltage regulation. When the mains voltage is low or high, a line interactive UPS corrects the output voltage by changing the taps of an autotransformer, and it does not use the battery while doing so. It switches to the battery only when the voltage goes outside the correction range or the mains is cut. This way the battery is used less on lines where the voltage changes frequently; the output frequency follows the mains.
How does the UPS battery runtime change as the load increases?
The runtime gets shorter as the load increases. Because the usable capacity of lead-acid batteries depends on the discharge rate, the runtime cannot be calculated by assuming an inverse proportion to the load; it is read from the manufacturer's load-runtime table for your own load value. When a longer runtime is needed, an additional battery module or an external battery bank is planned; the amount of battery that can be added is stated in the manufacturer data.
What are a battery module and an empty battery cabinet for?
A battery module is a ready-made battery bank added to the UPS; it is used to equip a device listed without battery with batteries or to extend the runtime. An empty battery cabinet is the enclosure in which additional batteries are placed; the batteries to be installed in it are planned separately. Each module and cabinet is made for a specific UPS series; the compatible series is stated in the product name or in the technical specifications. The number of modules that can be added is checked in the manufacturer data.
Why is the UPS output waveform important?
Some line interactive UPSs produce a stepped waveform instead of a sine wave in battery operation; in the Easy UPS BVS record in this category, the waveform is given as a stepped approximation of a sine wave. The two letters in the second part of the IEC 62040-3 class code indicate the waveform: the first letter is for normal operation and the second letter for battery operation; S means a sinusoidal output and Y means a non-sinusoidal output. For motor loads or loads sensitive to the waveform, follow the load manufacturer's recommendation.