Building energy efficiency is the measure of the ratio between a building's useful delivered service—thermal comfort, illumination, ventilation, and domestic hot water—and the total energy consumed to provide that service, expressed as a quantitative performance rating or compliance threshold. The concept encompasses the thermal performance of the building envelope, the efficiency of heating, cooling, and ventilation systems, the performance of lighting and appliances, and the overall energy balance calculated across the operational life of the structure

Building energy efficiency refers to the ability of a building and its energy-using systems to provide required services, such as thermal comfort, lighting, ventilation, and domestic hot water, with a reduced amount of energy input ^[1]. It should be distinguished from energy consumption, which refers to the actual amount of energy used by a building during a specified period. Energy efficiency therefore concerns the relationship between energy inputs and the services or performance delivered, whereas energy consumption describes the quantity of energy consumed ^[1][2]. Building energy efficiency is influenced by the thermal performance of the building envelope, the efficiency of heating, cooling, and ventilation systems, lighting and appliances, building operation, and occupant-related factors. Assessment may be based on calculated or measured energy performance and can consider delivered energy, primary energy, or other energy-related metrics

^[1]. The physical basis is the steady-state and dynamic heat-transfer balance: heat gains through solar radiation and internal loads are balanced against losses through transmission, ventilation, and thermal bridging, and the efficiency metric quantifies how effectively the envelope and systems minimize energy input while maintaining indoor environmental quality. Certification frameworks and calculation standards establish numerical procedures for predicting annual energy consumption, comparing designs against a reference building, and assigning performance grades based on measured or modeled consumption per unit floor area

^[2]. ISO 52000-1 provides a framework for assessing the overall energy performance of new and existing buildings using measurement or calculation. In practice, building energy efficiency is evaluated using indicators such as energy use intensity (EUI), expressed as energy consumption per unit of floor area and time; primary energy use per unit of reference floor area; and energy performance ratings or benchmark scores

^[2]. The concept is operationalized through energy performance certificates that compare predicted operational energy use to benchmark values, and through building energy standards that prescribe minimum envelope insulation levels, maximum system efficiencies, and allowable window-to-wall ratios ^[3].

. EUI can be calculated from measured energy consumption and used to compare a building with similar buildings, historical performance, or a defined target. Building energy standards and performance requirements may specify minimum envelope insulation levels, minimum system efficiencies, maximum allowable energy use, or other performance targets ^[4]. Energy performance certificates and benchmarking schemes similarly provide standardized approaches for reporting or comparing building performance.