NOVELTY - A solar water heating system (100) comprises a solar collector panel (102); a solar water heating component; a piping system (120); an expansion reservoir (104) comprising a fluid passage having a fluid inlet and a fluid outlet that are both fluidly coupled with the solar collector panel and the solar water heating component via the piping system; a deformable bladder that is disposed adjacent to the fluid passage and that includes at least one aperture; and a housing that encases the fluid passage and the deformable bladder.    USE - Solar water heating system.    ADVANTAGE - The system is lightweight and affordable, has low-pressure, is easier to ship and assemble, and is resistant to overheating and freezing damage. It has the ability to draw power from both a photovoltaic panel and the electrical grid. The solar collector panel is unglazed and formed partially from a relatively lightweight polymer material. As a result, it is more portable, affordable and easier to install then its metallic, glazed counterparts. The use of an unglazed, polymer solar collector panel also causes the heat transfer fluid in the panel to reach relatively lower maximum temperatures and pressures. This feature provides protection against overheating and eliminates the need to form the piping system and collector largely of metal.    DETAILED DESCRIPTION - A solar water heating system (100) comprises a solar collector panel (102); a solar water heating component; a piping system (120); an expansion reservoir (104) comprising a fluid passage having a fluid inlet and a fluid outlet that are both fluidly coupled with the solar collector panel and the solar water heating component via the piping system; a deformable bladder that is disposed adjacent to the fluid passage and that includes at least one aperture; and a housing that encases the fluid passage and the deformable bladder. The deformable bladder is arranged to regulate its internal pressure such that air is expelled out of the aperture from the deformable bladder when pressure in the fluid passage increases and air is drawn into the deformable bladder through the aperture when contraction of fluid in the fluid passage forms a vacuum in the expansion reservoir. INDEPENDENT CLAIMS are included for:    (1) system for back flushing a filter in a solar water heating system; and    (2) a method of regulating electrical power in a solar water heating system.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic flow diagram of a solar water heating system.    Solar water heating system (100)    Solar collector panel (102)    Expansion reservoir (104)    Water storage tank (112)    Piping system (120)    Power supply (124)