An Engadget article addresses a common driver concern: whether charging a phone via a car's USB port meaningfully wastes petrol. The piece concludes that, while phone chargers technically draw power from the engine via the alternator (and thus do consume some fuel), the effect is so minuscule it is not worth worrying about, especially compared with other in-car electrical draws like air conditioning.

Citing Bloomberg and How-To Geek, the article notes a phone charger typically draws just 5-15 watts and reduces fuel economy by around 0.03 miles per gallon—meaning a car getting 20 MPG would drop to roughly 19.97 MPG. By contrast, air conditioning can raise fuel consumption by up to 10%, according to Kwik Fit. The article also clarifies that charging a phone while the engine is running poses no real risk to the car's battery, since the alternator supplies the power, though charging for extended periods with the engine off could theoretically drain an old or already weak battery.