Water parking heaters only add a small amount of fuel, but the fuel savings due to the warm-up of the engine almost completely make up for the fuel consumed during the heating phase. If the engine cold start, you need to consume more fuel. If it is idle with the car, more fuel than the parking heater 5 to 10 times. Therefore, the fuel consumed by the parking heater is completely negligible.
Air heater can work during engine stop, far less than the engine fuel consumption.