Recommendation ITU-T L.1302 contains the energy efficiency assessment methodology for data centre and telecom centre, test equipment accuracy requirements, assessment period, assessment conditions and calculation methods. For data centre and telecom centre, it was divided into assessment methods for whole data centre /telecom centre efficiency and partial data centre/telecom centre. As main energy consuming infrastructure in data centre/telecom centre are power feeding system (power supply system) and cooling system, both system energy efficiency measurement methodologies are covered in this Recommendation. It will take advantage of methodologies and best practices currently in used or in development in networks and data centre/telecom centre. This recommendation aimed at reducing the negative impact of data centre and telecom centre through providing the methodologies of energy efficiency assessment. It is commonly recognized that data centre and telecom centre will have an ever-increasing impact on the environment in the future. The application of the assessment methods defined in this Recommendation can help owners and managers to build future data centres/telecom centres, or improve existing ones, to operate in an environmentally responsible manner.