There are two product offerings available in the UK for offices buildings: Design for Performance to drive energy efficient new buildings, and NABERS Energy for Offices ratings to measure how energy efficient existing buildings are.
NABERS Energy measures the efficiency of an existing office building and rates its in-use performance. The energy rating works by comparing the energy consumption of a building against a set of benchmarks that have been developed using actual data.
NABERS Energy rates the base building performance. This includes central services like heating and cooling systems, lifts and lobby lighting, allowing office buildings to be compared based on how the central services are being run to provide a comfortable working environment for users.
To get a NABERS Energy rating you need an Accredited Assessor to undertake an assessment of your building.
You can view the full list of NABERS Accredited Assessors here