The invention relates to a heater for a fluid storage tank and has for its objective the use of a renewable energy source as its primary source of power. The use of such as solar cells to power e.g. an immersion heater in a water storage tank has not hitherto been practical. The object of the invention is to enable effective use to be made from a low power renewable energy source to heat a fluid storage tank, an objective that is met by a heater pad (3) to cover a part at least of the height and periphery of the storage tank exterior, characterised in that said pad (3) comprises an outer layer (5) of insulation material with an inner layer (8) of heatable material secured thereto and a facing (9) of an appropriate adhesive material on the heatable material, said heatable material (8) being formed from a carbon polymer product, there being at least four transverse and parallel rails (10, 11, 12, 13), with an electrical connection (14, 15, 16, 17) at a respective one end of each rail to a low voltage source of power, one electrical rail (10) being disposed along one outer edge of the heatable material (8) on the pad and another rail (13) being disposed along the opposite edge of the heatable material, the other two electrical rails (11, 12) being disposed in close parallel relationship generally centrally of the heatable material, the rails (10, 13) at the outer edges of the heatable material having the same polarity, and the two generally centrally disposed rails (11, 12) having the opposite polarity.