A method for operating a refrigeration system having a refrigeration unit for providing temperature conditioned air to a temperature controlled space, an engine and an electric generation device, the method comprising the steps of: providing a battery system having a least one battery unit supplying electric power; and during a high cooling demand mode, operating the engine to drive the electric generation device for supplying electric power and simultaneously employing the battery system for supplying electric power to jointly power a plurality of power demand loads (42, 46, 48, 50) of the refrigeration unit, wherein the refrigeration unit includes a refrigerant compression device, a refrigerant heat rejection heat exchanger and an associated fan, a refrigerant heat absorption heat exchanger and an associated fan, and a plurality of power demand loads (42, 46, 48, 50) including the compression device drive motor, a refrigerant heat rejection heat exchanger fan drive motor and a refrigerant heat absorption heat exchanger fan drive motor, further including a controller operatively associated with the refrigeration unit, the controller operative to selectively operate the refrigeration unit in a high cooling demand mode, and wherein the step of operating the engine to drive the electric generation device and simultaneously employing the battery system to power the plurality of power demand loads (42, 46, 48, 50) of the refrigeration unit during a high cooling demand mode comprises the steps of simultaneously operating both the engine to drive the electric generation device and employing the battery system to power the compression device drive motor and/or employing the battery system to power the refrigerant heat rejection heat exchanger fan motor and the refrigerant heat absorption heat exchanger fan motor. providing a battery system having a least one battery unit supplying electric power; and during a high cooling demand mode, operating the engine to drive the electric generation device for supplying electric power and simultaneously employing the battery system for supplying electric power to jointly power a plurality of power demand loads (42, 46, 48, 50) of the refrigeration unit, wherein the refrigeration unit includes a refrigerant compression device, a refrigerant heat rejection heat exchanger and an associated fan, a refrigerant heat absorption heat exchanger and an associated fan, and a plurality of power demand loads (42, 46, 48, 50) including the compression device drive motor, a refrigerant heat rejection heat exchanger fan drive motor and a refrigerant heat absorption heat exchanger fan drive motor, further including a controller operatively associated with the refrigeration unit, the controller operative to selectively operate the refrigeration unit in a high cooling demand mode, and wherein the step of operating the engine to drive the electric generation device and simultaneously employing the battery system to power the plurality of power demand loads (42, 46, 48, 50) of the refrigeration unit during a high cooling demand mode comprises the steps of simultaneously operating both the engine to drive the electric generation device and employing the battery system to power the compression device drive motor and/or employing the battery system to power the refrigerant heat rejection heat exchanger fan motor and the refrigerant heat absorption heat exchanger fan motor.