Energy Metrics, Benchmarking, and Reporting
What this domain tests
This domain is about measurement discipline: choosing the right metric, computing it correctly, and reporting it without misleading the reader.
Core concepts:
- PUE = total facility energy / IT energy. It is a ratio of energy over a period, not a power snapshot, and it varies with IT load and weather. Report the measurement period and boundary.
- pPUE and partial PUE: PUE for a subsystem or zone, useful for mixed-use facilities.
- DCiE = 1/PUE, expressed as a percentage.
- Other metrics: WUE (water), CUE (carbon), ITUE, and IT power density (kW per rack).
- Benchmarking: compare like with like - similar climate, redundancy, and density. Industry surveys give ranges, not targets.
- Reporting integrity: state the boundary, the measurement method, the period, and whether values are measured or modeled.
Traps to watch:
- Computing PUE from instantaneous power readings and calling it annual PUE.
- Mixing boundaries - e.g., including office load in IT energy.
- Comparing a 2N facility in a hot climate with an N facility in a cool one.
- Presenting the best instantaneous PUE rather than a period average.
Worked example
Over one year, a facility uses 8,760,000 kWh total and 5,475,000 kWh for IT. PUE = 8,760,000/5,475,000 = 1.60; DCiE = 1/1.60 = 62.5%. If the IT load grows 10% with overhead held constant, PUE falls toward 1.55 - improvement in the ratio does not necessarily mean less total energy was used.