Abstract: A solar heating system for heating a pool of water includes a water bag formed of materials such that when it is not in use, it is deflated and compressed so as to occupy little space. When the water bag is in use it is inflatable and expandable so as to contain a substantial amount of water which can be heated by the rays of the sun absorbed by the water bag. A water bag embodying the invention can be formed of inexpensive materials (e.g., plastic, rubber or canvas). The water bag is very portable and can be set up (to collect water and heat it) quickly and easily. When not in use it can be folded or rolled up taking up very little storage space. It does not interfere with the use of the pool of water it is intended to warm. It can be set up any place, close to the pool of water or by using longer hoses it can be located away from the pool, where the best sunlight and solar heat is available.