The Uninterruptible Power Supply (UPS) is a device placed between the power mains and the PC (usually just the server in a network) that protects the computer from power problems ( outages, surges). It is basically a battery with some logic to correctly shut down programs and the computer, a surge suppressor, a filter, a charger and a DC-AC converter. For home and small office use, capacities up to 1000 kVA are typical, allowing for a power failure of 5 to 20 minutes (at 200-400 W). Manufacturer is e.g. APC (

The energy consumption of a 'true' UPS is certainly not negligable. Even small UPSs have an 'Online thermal dissipation' of 7 to 8 Watts (25-30 BTU/hr; 1 BTU=1055 J). As it is a continuous use (8760 hr/year), this comes down to 60 to 70 kWh/year. Energy consumption figures are readily available from manufacturers because they are needed to calculate the cooling load of the air conditioning.

For a bit larger SME office networks, where the UPS-devices might easily take up a few hundred kWh's per year, the Swiss Federal Office of Energy (SFOE) has made some studies and is now proposing an energy label for UPS systems, using a Q/E matrix to evaluate both the process-oriented quality criteria and the energy relevant parameters. (see SFOE claims that through power management some 45% savings are possible for the UPS devices. For Switzerland the SFOE estimates all the UPS devices in networks to consume 28 GWh/year on a total of 780 GWh/yr for all office network components (including mainframes, terminals, etc.).

And finally there is the case of the really large "class A" data centers. Katz (see describes a case of a 40.000 ft2 center in the US, using a total of 15 GWh (15.000.000 kWh) per year. A Standard UPS system for this center will have a heat loss of 1,75 GWh and will add some 0.57 GWh to the cooling cost (airconditioning) making the electricity consumption due to UPS some 2,32 GWh/yr (15% of total). With High efficiency UPS, the UPS heat loss would be 1 GWh and the cooling cost would add 0.33 GWh per year. This means a saving of 43% on UPS energy consumption and a cost saving of around 100.000 EUR/yr. per data center.

For more background information on the different types of UPS (standby UPS, true/online UPS, Ferro resonant standby UPS) see

Courtesy of the French Energy Agency ADEME, Future Electronics project.