First, let us consider the factors upon which the energy performance depends. From case studies, energy audits and studies using mathematical models, a picture is beginning to emerge which provides some explanation for this. Three types of factor can be identified, as in Figure 2.1:

1 building design; 2 services design and performance (systems); 3 occupant behaviour

A fourth factor could be the presence of a particular activity or process in the building, such as the use of a large computer in an office building, a heavy use of energy for a swimming pool in an institutional building, or the process energy in a factory. When assessing a building’s performance it is essential to be aware of these cases, and account for them separately from the energy for environmental conditioning.