An uninterruptible power supply (UPS) is a battery-powered electronic device that continues to supply electricity to the load for a certain period of time during a utility failure or when the line voltage varies outside the normal limits. Its typical application is PC backup power. Larger permanently wired devices can be used to feed servers and other equipment, or even an entire home. Besides the backup, most models also provide surge protection and power conditioning. The generic standard for UPS backup power supply systems is IEC 62040 series.

Even a brief power outage could result in loss of unsaved data on a desktop computer that was running or reset you modem. That's why an UPS is a must in home and office. From a technical standpoint, to make a power supply uninterruptible, you need an energy storage backup battery, an AC-DC charger and a DC-AC inverter.


. There are three main types of UPS: Standby (off-line), Line Interactive and Online. Each of these types supplies energy from the battery when the grid fails, but under normal conditions they process the power differently. Note that unlike home generators, none of them needs any moving parts.

A Standby UPS (SPS) includes a transfer relay. It switches the load to the battery-powered inverter when the primary AC is out of range. A typical transfer time is between 2 ms and 10 ms depending on the amount of time it takes to detect the lost utility voltage and turn on the inverter. During this time the current to the load is momentarily interrupted. That's why listing such devices as "uninterruptible" is somewhat misleading. Today's PC include an internal switching-mode PSU which inherently provides certain hold up ("ride through") time. For a reference, SMPS PSU for personal computers are required to provide at least 16 ms hold-up time at rated load (this number corresponds to one cycle of 60Hz input). Since it is greater than normal UPS transfer time, most PCs are not affected by a transition from mains to UPS.
On the positive side, since SPS inverter operates in standby mode and starts up only when the input source fails, it has the highest efficiency (95-98%) and reliability. Because it is also the cheapest UPS power supply (under $0.10 per rated volt-amp), it became the most common backup product used for desktop PCs. Note, in some older systems the inverter generated square-wave type output rather than a sinusoidal one, which could cause problems to some sensitive equipment. An example of a popular standby model with good reviews is CyberPower CP750LCD.

The Ferroresonant type of standby UPS has an additional ferroresonant transformer that shapes output voltage and stores some energy for a smoother transfer. Its main drawback is instability when it is loaded by an SMPS with a PFC front end. For this reason such systems are no longer commonly used.

A Line Interactive UPS under normal conditions smooths and to some degree regulates the input AC voltage by a filter and a tap-changing transformer. The bi-directional inverter/charger is always connected to the output and uses a portion of AC power to keep the battery charged. When the input source fails, the transfer switch disconnects AC input, and the battery/inverter then feeds the load. The typical efficiency of this type is 90-96%. It is currently the most common design in 0.5-5 kVA power range. An example of Line Interactive type is Tripp Lite AVR550U.

An Online UPS always delivers all or at least a portion of the output power through its inverter even under normal line conditions, and therefore provides true uninterruptible power with 0 ms transfer time. An example of online type is CyberPower OL1000RTXL2U. This type is more expensive. Its main applications are small servers and networks, but nowadays it is used in homes as well. There are two main types of on-line UPS: double conversion and delta conversion.

A Double Conversion online UPS is continuously processing the whole power through a series connected AC-DC rectifier/charger and DC-AC inverter. An additional bypass switch allows you to support the loads directly from an AC source under some fault conditions (such as the inverter failure). Although such a type provides PFC and better output voltage quality than other designs, the double energy processing results in reduced efficiency (80-90 % typical). This type is common for critical applications.

A Delta Conversion online UPS includes an additional "Delta Converter" that delivers a portion of the energy directly to the load and provides power factor correction. Such partial bypassing the rectifier / inverter stages during normal operation results in higher efficiency (up to 97%).

For most desktop applications an SPS type is sufficient. However, if you want to avoid even brief power interruptions, look for a line interactive type. Among main domestic suppliers Tripp Lite probably is the only one that clearly states the types of its models. Various performance and test requirements, such as the limits on the amplitude and duration of deviation of the output voltage acceptable for switching mode (SMPS) PSU loads are defined by IEC 62040-3 standard. Note that manufacturers of small commodity UPS power supplies for PCs usually advertise their systems by volt-ampere (VA) rating. A typical maximum real power in watts of such backup power supply is only 60% of its nameplate VA rating. Such default ratio between watts and VA is based on an old non-PFC computer PSU that used to have power factor between 0.6 and 0.7. So, when you are buying a UPS power unit, make sure that the net wattage of your loads does not exceed 60% of the its VA rating. To find your required wattage add nameplate currents of all devices you want to backup and multiply the result by 120. Note that technically, this will give you volt-amps rather than watts. However, today's electronics have nearly unity power factor. So, your watts will be almost the same. For example, if your system draws 2.5 A, then it consumes up to 2.5x120=300 watt. In this case you need to select a backup device rated for at least 300/0.6=500 VA.

Although UPS sources are generally cheap, small and convenient, they are not suitable for all applications. Their common disadvantage is a relatively short runtime. That's why most data sheets state it at half load. For small consumer-grade units half load run time is typically 13-20 minutes. Note that this characteristic is not linear. At full load you may get only 1/3 of half-load life. If you are looking for a longer backup consider electric generators.



Types and specification of uninterrupted systems

Selecting a UPS for stand-alone single-phase computer backup power

DIY: how to build your own uninterruptible power supply


UPS reviews and buying guide

Best models- four devices comparison and tests


Belkin F6C550-AVR 550VA

Tripp Lite 550SER 550VA

Reference design of an offline UPS with a microcontroller and AC sine wave generation

APC UPS 250_400_600 schematic (download and save the file- it opens only locally)

The designer's guide to optimizing uninterruptible power supply for modern data processing equipment

SBS and On-line UPS: overview and tutorial