Abstract: The present invention discloses a solar array cell (15) able to be used both thermally and photovoltaically, together with an array (3) formed from a plurality of the cells (15). A solar energy system for a building which incorporates the array (3) is also disclosed. Each cell (15) is formed from an air duct (16) of parallelogram cross-sectional shape which makes for easy sealing between ducts and a reliable water shedding arrangement for the cells of the array. An air/liquid heat exchanger (35) for a solar hot water supply is also disclosed.