An APU (Auxiliary Power Unit) is a small engine that is attached to the side of a semi-truck and runs off diesel fuel or electricity. It provides power to the trucks cab to keep it cool in warm months and to keep the battery charged and the engine block warm in cool months. The APU can also power up halogen lamps, heating and cooling systems, and interior appliances without the need to run the trucks engine. The most basic function of an APU is to cool the sleeper cab while a driver rests. APUs are beneficial additions to trucks because they allow drivers to maintain a comfortable temperature in their sleeper berth without having to idle their truck, which not only uses excessive fuel but can also be hard on the engine and the emissions systems. There are two basic types of APUs: diesel-powered and battery-powered. Diesel APUs are frame-mounted and run off the fuel supply from the truck, while electric APUs are battery-operated and provide a silent operation. The cost of APUs ranges from $6,000 to $13,000, including installation.