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ITGlossary · UPS technology
Rack Unit (U)
A standard height unit for equipment in 19-inch racks, where 1U equals 44.45 mm.
Technical level:Basic
The height of equipment in a 19-inch rack is measured in units (U), where 1U equals 1.75 inches, or 44.45 mm. A patch panel or switch usually takes up 1U, a server 1U to 4U, and a rack-mount UPS often 1U to 3U, plus any external battery modules.
Rack cabinets come in sizes such as 12U, 22U, 42U or 47U. To choose the right one, add up the U of all your equipment and allow extra space for cable managers, power strips, shelves and airflow.
A practical tip: leave about 20–30% free space for future expansion, and place the UPS and batteries at the bottom of the rack, where their weight keeps the cabinet stable.
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Related terms
- Offline (Standby) UPSA UPS that leaves the load on the mains and switches to battery only when the mains fails, with a short transfer time; the simplest and cheapest topology.
- Rack-Mount UPSA UPS designed to fit a standard 19-inch rack, with its height measured in rack units (U).
- Capacity SystemParallel UPS units that share the load with no spare unit (N configuration), so the failure of one unit overloads the rest.
- Modular UPSA UPS design where independent modules can be combined to meet power and redundancy needs.
- PDU (Power Distribution Unit)A rack power strip that distributes UPS power to the equipment, optionally with metering or remote control.
- Pure Sine WaveUPS output with the same waveform as the mains, needed for PFC power supplies, motors and sensitive equipment, unlike a stepped approximation.