PUE (Power Usage Effectiveness) is the most widely used metric to evaluate the energy efficiency of data centers. Defined by The Green Grid and adopted in ISO/IEC 30134-2, PUE is the ratio of the total facility power to the ICT (IT load) power.
Formula:
* Total Facility Power includes all electrical consumption: IT, cooling, lighting, power distribution losses, UPS inefficiency, etc.
* ICT Equipment Power is only the load drawn by servers, storage, and networking gear.
An ideal PUE is 1.0, meaning all power is used by ICT equipment with no overhead. Typical enterprise values are 1.5-2.0, while hyperscale operators target <1.2.
Other options are incorrect:
* B represents the inverse metric, known as DCiE (Data Center infrastructure Efficiency).
* C and D are partial subsystem efficiency metrics, not the global PUE.
References: ISO/IEC 30134-2 (KPIs - PUE), The Green Grid White Paper #49, ANSI/TIA-942-B §7.3.