Energy efficient capacity control for an air conditioning system

An air conditioning system (10) is provided for cooling a compartment (16) of a truck when the main engine is not running. The system (10) includes a variable speed compressor (20), a variable speed condenser fan (22), a variable speed evaporator blower (24), and a controller (14) configured to optimize the cooling capacity of the system (10) to the cooling requirements of the compartment (16) by selectively adjusting the speeds of the variable speed components (20, 22, 24).