Heat gains by inner and outer skins were estimated from the heat balance between solar radiation, heat transmission, absorption and reflection as well as convection and long-wave radiation (Fig. 13). For example, for glazing with an absorptivity for direct solar radiation of 0.08, transmittance of 0.84 and reflectivity of 0.08, based on the maximum solar radiance on a vertical south wall in London at 12:30 29 January of 740 W/m2 [21], the estimated heat absorption rates by the outer and inner glass skins were approximately 21 and 25 W/m2, respectively. When the glazing for the outer skin was replaced in part or whole by PV cells/modules, i.e. fac¸adeintegrated PV system, electricity conversion was also taken into account for the heat balance.