An Uninterruptible Power Supply, or UPS System (or simply UPS) is a device that provides continual power to connected equipment in the event that the incoming mains supply fails (a power cut), or goes out of tolerance (a surge or a brown out).
They do this by converting stored energy, usually from a battery (a DC source), into useful power that your equipment uses (an AC source) through an electrical circuit called an inverter. For this reason they are also referred to as battery backups. Note that unlike one of our confused customers this does not mean they backup data, this is something else entirely!
UPS are used on electrical systems where the consequences of a power disturbance can lead from simply being annoying to the loss of revenue, time or even life.
The amount of power a UPS can deliver is limited and so each UPS has a power rating. This is usually specified in Volt-Amps (VA), but this can be (and is) misleading, and so the UPS will also have a rating in Watts (W). This is the real rating and limits what you should connect to the UPS.
The UPS will also contain batteries, or is connected to external batteries, either in compatible cabinets or a bespoke battery string. The amount and type of battery determines the amount of runtime you can expect when the unit is operating from battery power.
There are 3 classes of UPS that determine the level of performance and against what power disturbances they will protect your equipment against. Each has pros and cons over the other. In addition to the UPS class, there are also two types of inverter. These types of inverter are categorised by the power waveform they produce, either sine wave, or non-sine. A sine wave inverter produces a power output similar if not better than is provided by your standard mains outlet. A non sine inverter produces an output that is only suitable for certain types of computer power supply. These non sine inverters are called by various names such as pseudo-sine, modified sine, stepped wave etc. We just call them square wave.
To learn more about the different types of UPS and how to choose the correct system for your application, please see the Further Information links.