AT-19-C057 -- Comparison of Approaches for Calculating Annualized Data Center Energy Metrics

Calculation of the annualized Mechanical Load Component (MLC) and Power Usage Effectiveness (PUE) of a data centerrequires determining the annual facility energy consumption in kWh. For this purpose, ASHRAE Standard 90.4 recommends runningan annual simulation for 8760 hours using an hourly timestep. The Standard also allows an annualized metric calculation by running asimulation with external temperature values placed into bins with a 1°C (2°F) increment. However, the influence of bin size in thelatter approach on the calculated annualized MLC and PUE is unknown. Therefore, this paper examines the influence of bin size onthese energy metrics for four commonly-encountered climate conditions in the US. The results of this work show that the calculatedmetrics are essentially independent of bin size. A minimum bin width limit of 0.5-0.75°C (0.9-1.35°F) is seen for all four locationswhere non-continugous bins occur due to the hourly discretization of annual weather data. A maximum bin width limit also exists thatresults in nonphysical psychrometric conditions for the chosen binning method, and this limit is location-dependent since these nonphysicalconditions occur near saturation conditions.