Energies 2015,8 10997 occupants'energy-use characteristics.Based on the detailed review of each approach, critical issues and current gaps in knowledge in the existing literature are discussed,and directions for future research opportunities in this field are provided. Keywords:commercial building;energy consumption;occupant energy use behavior; occupancy related approaches;review 1.Introduction The world's growing energy use raises concerns about energy consumption and its impacts,particularly in terms of resource consumption and environmental degradation.In the last two decades,global energy use has increased by 50 percent,and current predictions show an increasing trend of 2 percent in annual global energy consumption [1,2].Currently,residential and commercial buildings share 40 percent of this total global energy consumption [3]and are responsible for a similar percentage of CO2 emissions [4,5]. Such facts are particularly visible in the United States and European Union,where total energy-use in built environments is more pronounced than in other major energy end-use sectors-e.g.,industry and transportation [2,3].Contributing to this rising building energy use are population growth,increasing demand for maintaining a comfortable environment,and increasing time spent inside of buildings [2]. These factors point to the significance of residential and commercial building sectors in energy consumption [6,7].The commercial building sector currently consumes about 12 percent of global energy use and 21 percent of United States'total energy use [3].Its energy use intensity (energy per unit floor area per year)increased by 12 percent [8],and it has the greatest intensity rate when compared to residential or industrial sectors [9].In addition,the energy demands of the commercial sector currently has an increasing rate of 2.9 percent and continues to grow faster than other major sectors:industry, residential buildings,and transportation [3,10].Such energy use intensity and its increasing rate raise a critical concern about improving the energy performance of commercial buildings,which has brought about a greater emphasis on the importance of maximizing energy savings during the operational phase The need for improved operational efficiency has attracted attention from industry,research,and government to address energy saving approaches.Overall energy consumption in buildings during the operational phase generally depends on four main characteristics [2,11-17]:(1)climate characteristics (2)the building's physical characteristics;(3)appliances'and systems'characteristics and;(4)occupants' energy behavior characteristics.Improving climate characteristics is not possible at a given location. Enhancing the building's characteristics (building envelope)and appliance and system approaches require large capital investments and sometimes are infeasible for existing commercial buildings [13] This leaves occupants'energy behavior characteristics as a prime target for energy conservation [18-20]. The commercial built environment's energy use is highly connected to the energy-use behavior of its occupants [21-23].This behavior includes individual occupant's presence in a building and such occupants' actions and interactions that influence the energy-use of the building [24].These occupancy actions and interactions use up to 70 percent of the United States'total electricity of built environments [25]. A single occupancy-driven energy parameter-e.g.,heating,ventilation,and air conditioning (HVAC) set-pointscan impact building energy performance up to 40 percent [26,27],and uncertainties inEnergies 2015, 8 10997 occupants’ energy-use characteristics. Based on the detailed review of each approach, critical issues and current gaps in knowledge in the existing literature are discussed, and directions for future research opportunities in this field are provided. Keywords: commercial building; energy consumption; occupant energy use behavior; occupancy related approaches; review 1. Introduction The world’s growing energy use raises concerns about energy consumption and its impacts, particularly in terms of resource consumption and environmental degradation. In the last two decades, global energy use has increased by 50 percent, and current predictions show an increasing trend of 2 percent in annual global energy consumption [1,2]. Currently, residential and commercial buildings share 40 percent of this total global energy consumption [3] and are responsible for a similar percentage of CO2 emissions [4,5]. Such facts are particularly visible in the United States and European Union, where total energy-use in built environments is more pronounced than in other major energy end-use sectors—e.g., industry and transportation [2,3]. Contributing to this rising building energy use are population growth, increasing demand for maintaining a comfortable environment, and increasing time spent inside of buildings [2]. These factors point to the significance of residential and commercial building sectors in energy consumption [6,7]. The commercial building sector currently consumes about 12 percent of global energy use and 21 percent of United States’ total energy use [3]. Its energy use intensity (energy per unit floor area per year) increased by 12 percent [8], and it has the greatest intensity rate when compared to residential or industrial sectors [9]. In addition, the energy demands of the commercial sector currently has an increasing rate of 2.9 percent and continues to grow faster than other major sectors: industry, residential buildings, and transportation [3,10]. Such energy use intensity and its increasing rate raise a critical concern about improving the energy performance of commercial buildings, which has brought about a greater emphasis on the importance of maximizing energy savings during the operational phase. The need for improved operational efficiency has attracted attention from industry, research, and government to address energy saving approaches. Overall energy consumption in buildings during the operational phase generally depends on four main characteristics [2,11–17]: (1) climate characteristics (2) the building’s physical characteristics; (3) appliances’ and systems’ characteristics and; (4) occupants’ energy behavior characteristics. Improving climate characteristics is not possible at a given location. Enhancing the building’s characteristics (building envelope) and appliance and system approaches require large capital investments and sometimes are infeasible for existing commercial buildings [13]. This leaves occupants’ energy behavior characteristics as a prime target for energy conservation [18–20]. The commercial built environment’s energy use is highly connected to the energy-use behavior of its occupants [21–23]. This behavior includes individual occupant’s presence in a building and such occupants’ actions and interactions that influence the energy-use of the building [24]. These occupancy actions and interactions use up to 70 percent of the United States’ total electricity of built environments [25]. A single occupancy-driven energy parameter—e.g., heating, ventilation, and air conditioning (HVAC) set-points—can impact building energy performance up to 40 percent [26,27], and uncertainties in