The fuel consumption of a car when it is idling with the air conditioning (AC) on can vary depending on several factors, including the car's make and model, engine size, AC efficiency, outside temperature, and how long the car remains idle. Generally, idling with the AC on consumes more fuel than simply turning off the engine and restarting it later.
As an approximate estimate, an average car may consume around 0.1 to 0.5 gallons (0.38 to 1.89 liters) of fuel per hour when idling with the AC running. However, keep in mind that these numbers can vary significantly based on the factors mentioned above.
To minimize fuel consumption and save energy, it's recommended to turn off the engine if you expect to be idling for more than a minute or two, especially in situations like waiting in a long queue or parked in a stationary position with the AC on. Modern cars are designed to start efficiently, so the fuel consumed during startup is generally less than the fuel consumed during prolonged idling.
Remember, excessive idling not only wastes fuel and increases emissions but also puts unnecessary wear on the engine and other components. If you're concerned about the environmental impact or fuel efficiency, it's best to turn off the engine when you know you'll be stationary for an extended period.