This report proposes a definition and characterisation of data centres, presents trends in their energy use, and analyses the availability of data which might be used as inputs to energy efficiency metrics. It provides an analysis of the parameters that have the highest influence on data centre energy demand. The report indicates that reliable, publicly available information on the energy used by data centres is scarce and suggests further efforts towards assembling a complete and continuously updated data set with the following information for data centres: location; data centre type (enterprise/co-location/co-hosting); energy use and PUE; and contact information. The report also flags the need for further efforts to aquire data on the operational efficiency of data centres.