Uninterruptible Power Supplies
UPS units serve multiple purposes, and they come in different form factors.
Enterprise-class devices are usually created for 19” rackmount environments; this is the case for the device we reviewed. However, there are many pedestal units available as well, which can be positioned right next to the systems they are intended to support.
All UPS unit share (or at least should share) a common characteristic, which is substantial weight as a result of the batteries used to provide backup power. Although there are much more capable battery solutions, lead-acid batteries are still most commonly used because they are affordable, reliable, and pretty predictable. In addition to lithium-ion technology, they are safe, as they cannot catch fire.
An uninterruptible power supply is meant to maintain power in case of a power outage. Depending on the battery capacities and load, different UPS devices can bridge a certain amount of load for a particular amount of time. Some UPSes are designed to provide just enough power for a short power outage, to allow administrators or the UPS software itself to shut down systems safely. Others are designed to bridge a longer period of time, although you’ll have to get a diesel generator or similar to be able to run servers truly independently from the power grid. Almost all UPS units come with surge suppressors to protect attached devices, and most also come with hardware to filter the electric current in an effort to maximize the power “quality.”
Devices typically have one power input and several outputs. The input has to be connected to the power grid, while the outputs can be used to connect client devices. Some outputs provide only surge suppression and filtering, while those providing backup power may also be divided into different segments for different buffering scenarios. The Powerware PW5130 unit even has a C19 output, which is good for up to 16 A, while the other C13 outputs (three pin power connectors) serve up to 10 A. Some UPS units also have ports to protect network paths from surges, but this isn’t the case for our review unit.
Professional UPS devices may use a rectifier (double conversion online UPS) or a transformer (delta conversion) in front of the battery, but this applies to enterprise-grade UPS devices, which we aren’t interested in at this point. The two most popular types are standby UPS units, which remain in standby mode and only switch to the battery if main power fails, and line-interactive designs. The latter is based on having the AC/DC inverter as part of the active circuit at all times.
Battery Backup Basics
Once you know the functional details and features, you’ll have to pick a UPS unit that will be powerful enough for your type of application. The first step is the power rating, which is typically specified in VA (volt-amps). It is important to know that these ratings consist of effective power and reactive power, hence the number does not reflect the actual maximum power the UPS device can supply. You have to look into the product’s data sheet to find out the difference. In the case of the Eaton/Powerware unit, this is 2,700 W based on a 3,000 VA total power rating. The data sheet will then tell you how long the battery will run based on a certain load.