A system and method for controlling swimming pool or spa heaters is provided in the present invention. The system and method control the temperature of the pool or spa water and saves energy by heating the water therein only when future weather conditions indicate that it will be feasible and cost effective to do so in an automated process that depends upon forecast and actual weather and measured data at the pool. The system and method also utilize prior actual experience in heating the pool or spa to subsequently improve energy savings over time.