It's not just speeding, traffic, or hard acceleration that hurts your fuel economy. Many in-car features get switched on almost automatically, but each one draws power—and that means the engine has to burn more fuel.
The air conditioner is the most obvious culprit. According to ADAC data cited by SPEEDME, running the A/C in the city can increase fuel use by up to 20 percent. For a car averaging 5 L/100 km, that's an extra liter every 100 km. On the highway, the penalty is lower—around 6 percent, or 0.3 L/100 km. On average, air conditioning adds 10 to 15 percent to consumption. Still, it's not worth turning it off completely to save fuel: fogged windows, heat, and driver fatigue can be far more dangerous than a little extra gas.
Electrical accessories don't come for free either. The battery is charged by the alternator, which puts a load on the engine. ADAC offers a simple rule of thumb: every 100 watts of electrical consumption adds roughly 0.1 L/100 km to fuel use. That means the front or rear window defroster, drawing around 800 watts, can add up to 0.8 L/100 km. The cabin fan on medium requires about 170 watts, low beams 125 watts, fog lights 110 watts, seat heaters roughly 100 watts, and a heated steering wheel 50 watts.
Some systems are even thirstier. An auxiliary heater can consume up to 2000 watts, translating to as much as 2 L/100 km. Against that backdrop, the radio's 20 watts, a USB port's 100 watts, or a portable navigation unit's 10 watts have almost no impact on the wallet—even if they technically still add a load.
The key is sensible, not fanatical, economy. Air conditioning, defrosters, and lights are essential for safety, but there's no reason to leave high-power accessories running out of habit. Sometimes the extra fuel consumption starts not from the gas pedal but from a button that was long overdue to be switched off.