A vehicle engine stop/start system with regenerative braking comprising a main engine for powering a vehicle having an accessory belt drive system, a motor/generator engaged with the accessory belt drive system, an operator activated switch for selecting an operating mode, in a first operating mode an electric energy storage portion for providing electric power to a heating system when the main engine is off, a relay for preventing operation of the heating system when the main engine is running, in a second operating mode the electric energy storage portion providing electric power to the motor/generator whereby the motor/generator drives an air conditioning compressor when the main engine is off, the air conditioning compressor connected to the accessory belt drive system, a relay for preventing operation of the air conditioning compressor using the motor/generator when the main engine is running, in a third operating mode the motor/generator is used to start the main engine after the main engine has been automatically stopped, and in a fourth operating mode during operation of the main engine the motor/generator generates an electric current during vehicle braking whereby the electric energy storage portion is recharged.