Power Usage Effectiveness (PUE) and Data Center Infrastructure Efficiency (DCiE) are commonly used metrics for determining if energy efficiency improvements should be made.
PUE = Total Facility Power / IT Equipment Power
The range is from 1 to infinity (5 in Data Center Operation: Energy Efficiency). The lower the value is, the better the efficiency is. 1 being 100% efficiency (all power used by IT equipment only).
DCiE = IT Equipment Power / Total Facility Power x 100%
The range is a percentage. The higher the value is, the better the efficiency is. 100% being 100% efficiency (all power used by IT equipment only).
*Total Facility Power is the total power entering the data center. The measurement point is at or near the facility's utility meter.
*IT Equipment Power is the power delivered to the servers (after power conversion, switching, etc.).