A solar panel suitable for panelized building construction as an integral part of a roof is described. The solar absorber plate of the panel consists of a thermally conductive sheet folded along a plurality of parallel lines to form sharp vees or corrugations. The folded solar absorber plate is attached to or formed as part of a flat sheet, also with high thermal conductivity. The folded absorber plate with its vee-shaped corrugations in combination with the flat sheet form a plurality of contiguous triangular ducts through which the fluid to be heated flows. The fluid is in direct contact with the solar absorber plate and the flat thermally conductive plate which together form the triangular ducts. A pliable insulating board may be provided as a backing member to the triangular duct assembly, said insulating board serves as an elastic mounting means for the solar panel on a building roof or wall. The triangular ducts may be oriented in an East-West direction to maximize the number of reflections of solar radiation between the ducts
