According to the climate action plan published by the Environment Bureau in Hong Kong, it is expected that the absolute carbon emission to have a reduction of 36% per capita by 2030 (base year 2005). Using walkthrough combined with energy simulation, we have investigated the energy consumption pattern of a student residential hall. Due to the fluctuation nature on the number of residents in a student residential hall, we have simulated several scenario: 1) typical occupancy, 2) typical occupancy with energy optimization, 3) peak occupancy, 4) peak occupancy with energy optimization in order to estimate the range of consumption. In scenario 4, effective energy use pattern for air-conditioning system was implemented and approximately 480 tonnes (6.5%) of carbon emission can be reduced for a 50 years building life span consideration. Various energy conservation measures should be taken into consideration in order to further reduce carbon emission in the future. © Published under licence by IOP Publishing Ltd.