Whenever a vehicle is operating, all the electrical systems need power and that power comes from the alternator. The alternator is driven by a belt from the engine and, like anything else driven off the engine, the alternator puts a load on the engine that consumes power. Any power consumed by the various accessory drives is not available to move the vehicle. In order to maintain vehicle performance while adding accessory loads, more power is needed which means more fuel burned.
One obvious way to reduce vehicle consumption is to reduce the various parasitic losses on the engine. That's why car makers are switching from using constant mechanically-driven hydraulic steering assist to electrically-driven systems that only draw power when needed. One of the biggest users of electrical power in the vehicle is the lighting systems, both interior and exterior. Ford technology specialist Mahendra Dassanayake is working on reducing the power draw of lighting systems which he says account for up to five percent of a modern vehicle's fuel consumption. A lot of his present efforts are focused on LED lighting, which requires much less power to produce the same amount of light as incandescent or halogen lights. The Detroit Free Press has a profile of Dassanayake's work.
[Source: Detroit Free Press]