Many electric utilities are investigating and promoting the use of cool storage to reduce peak demand and increase the use of off-peak electricity. Building owners installing cool storage can reduce utility bills by shifting electricity consumption away from expensive peak periods and into off-peak periods when electricity costs and demand charges are lower. Data collected by EPRI indicate that many systems are operated at less than obtainable efficiency because of the lack of proper controls. This led to the developmeonft an optimal controller for cool storage systems. This paper describes the results of that development, which is a controller presently on field test in Montgomery, AL.