Abstract: A solar heater adapted to be made as an integral, sealed unit for installation in a building exterior wall or for use in an existing window opening or the like. A collector is operative to transfer solar heat to an adjacent evaporator to heat and vaporize a liquid refrigerant. The refrigerant vapor rises naturally into a condenser disposed in intimate contact with a solid phase change material carried in a plurality of spaced containers. The refrigerant is condensed and flows by gravity back to the evaporator, giving up its heat of vaporization to the phase change material, resulting in melting of a portion of the material, and thereby storing heat. Air circulating past the containers receives heat from the material for heating the building interior during the night as well as the day. Stackable, nestable units are disclosed.