The pursuit for a holistic metric that covers the complete data center performance is still ongoing. Data center efficiency encompasses electricity entering the data center and a fraction of that electricity being used to power vital IT equipment. Very much like comparing real and apparent power, savings are possible by improving airflow for cooling, adopting unique and free cooling methods, upgrading cooling equipment, and utilizing renewable energy all managed and controlled by an intelligent energy management system. Efforts are being made constantly to cut energy used by IT equipment by upgrading to lower power CPUs, using more efficient hard drives, use of virtualization technology, etc.
Power usage effectiveness (PUE) and data center infrastructure efficiency (DCiE) have been popular tools for benchmarking efficiency. This has led to the well-known “PUE Wars.” The Green Grid, author of both PUE and DCiE, didn't aim for these metrics to be used for comparing one data center to another. While this is a good starting point, it is important to understand that these metrics by themselves fall short in determining data center efficiency as they don’t include productivity.