An air conditioning system for use in an over-the-road or off road vehicle is provided that provides a method of operation during both engine on and engine off conditions. The method operates the air conditioning system at one capacity when the engine is running, and at a second capacity when the engine is not running. The selection of these capacities is based on the power capacity of the source of electric power from which the air conditioning system is operated. When a storage battery is used to power the air conditioning system during engine off conditions, the second capacity is lower than the capacity at which the system is operated when the engine is running.