Does a Car Burn More Fuel When the Lights Are Turned On?

Fuel efficiency will vary with the demands placed on a car's power plant.

Electricity for a car's lighting, as well as for all electrical systems, is provided by a device in the engine compartment called an "alternator," which is turned by an engine-driven belt. The power load on the alternator will affect fuel efficiency--but minimally.

Electrical Energy Generated by an Alternator

The alternator at the center of the photo supplies energy to the car's electrical systems.

The alternator provides electricity in the form of alternating current (AC), hence the name "alternator." This electrical current fires the spark plugs that ignite the fuel and oxygen mixture in the engine's combustion chamber that drives the piston that turns the crankshaft. A mechanical device bolted to the engine, the alternator is turned by a belt driven by the energy relayed to the flywheel by the engine's crankshaft.

Fuel Efficiency Depends on Engine Load

If the alternator provides electrical energy to power your car's on-board systems, then any increase in those energy demands will translate into lower fuel efficiency, because greater energy is required by the internal combustion engine to turn the alternator, which in turn requires more fuel to power the engine. Changes in fuel efficiency from the use of electrical systems and devices in a car, however, will be negligible.

Sample Electrical Systems

Examples of a car's electrical systems include headlamps, heater fan, rear window defroster, radio, navigational system, dome lights and power windows.

References

• Free Dictionary: Alternator
• Daily Fuel Economy Tip: Reduce the Use of Electrical Devices