There are two key things to think about when selecting/specifying a UPS system:
The amount of power you need the UPS system to supply. This is normally specified in VA or Watts.
It is often impossible to accurately predict future requirements, therefore it is important to oversize the UPS you choose. The reason for this is that any UPS you buy will be expected to last at least 5 years, and larger units should be expected to last for anything up to 15 years.
When considering the size of UPS, it’s not the power rating of the UPS electronics that makes a big jump in price, it’s the cost of the batteries!
UPS back-up times vary from about 5 minutes to many hours.
The main question to ask before deciding on the UPS backup time is:
In the event of a long power outage “what do we want to do”?
Depending upon the UPS size, if you are considering a long autonomy, it may be more economical to install an emergency generator, which will run for as long as it has fuel. You could then reduce the UPS battery back-up time and offset the battery cost saving against the generator cost.
Computer room air-conditioning units are not normally powered by the UPS, and the server room may overheat quite quickly during a mains power failure. An emergency generator could power the air-conditioning units and the UPS.
Depending upon the location of the UPS, remote communications may be required to ensure the user is fully aware of the UPS operational status.
All UPS’s provide audible and visual alarm signaling, however, should the UPS be remote from the user, i.e. in a computer room or plant room, it is important that key personnel are kept informed.
Most UPS systems come with an RS-232 port for local communications with a PC, however, options for many systems include:
To aid UPS communications with building management systems (BMS) and other control devices, there are a variety of adaptors available, such as MODBUS/JBUS, RS-485, Profibus DP etc.