As its name suggests, Solar XXI was created with the objective of making extensive use of solar exposure. The building is constructed to provide high internal thermal capacity with 5 cm expanded polystyrene external insulation to both walls and roof slab to reduce heat conduction gains and losses. The main façade faces south and contains the majority of the glazing as well as supporting 100 m² of photovoltaic panels. The glazing arrangement is intended to optimise passive solar gain in the heating season. Additional space heating is provided by a roof-mounted array of 16 m² of CPC solar collectors which heat water supplying radiators as well as domestic hot water. The hot water supply from the solar collectors can, when necessary, be supplemented by a condensing gas boiler. Electricity is supplied by the 100 m² of photovoltaic panels mounted on the south façade and an additional array of panels located to provide shelter and shade in the nearby car park. The total installed peak power is 18 kWp. When necessary the electricity supplied by the PV panels is supplemented from the national grid.Solar XXI has no active cooling system and a number of design measures are incorporated to reduce the summertime heat load. Venetian blinds were placed outside the glazing to limit direct solar gains. Natural ventilation during favourable conditions is promoted through the use of openings in the façade and between internal spaces, together with openable clerestory windows at roof level. When these methods are insufficient, incoming air can be pre-cooled by being drawn by small fans through an array of 32 underground pipes.. Each pipe has a diameter of 30 cm, length of 20 m and is buried at a depth of 4.6 m. Attention has been paid to the use natural lighting where possible. In the centre of the building there is a skylight that provides natural light to the corridors and north-facing rooms located on all three storeys. The installed artificial lighting load is 8 W/m².