Comparing the production and demolition stages in a building's lifecycle, the operation stage shares a much longer period, and the operation energy use contributes more than 80% of total energy consumption (Ramesh et al., 2010). Therefore, most co-benefits of energy saving, carbon reduction and costs cutting should be achieved in this stage. In order to achieve co-benefits, more attention should be put on the assessment of energy performance in this stage. Generally, per square meter energy use and GHG emissions of HVAC systems, lighting system, equipment and appliances in a certain period within the metered building/buildings are widely used for making the assessment and comparisons (Jiang, 2009). This kind of evaluation can be made by using the following equations.