Air conditioning compressors in a vehicle can use approximately 5-10 horsepower. They cycle and don't run all the time so the power consumed is not 100% of the time.

It is impossible to quantify how much fuel is consumed. For instance for a large motor, 5-10 horsepower is insignificant while a small motor, the 5-10 horsepower load can be significant. That is because motors consume more fuel under load. For a large motor, a 5-10 horsepower load is virtually nothing. Also, the amount of area the car needs to condition is different for each vehicle. The amount of insulation is different for each vehicle. The efficiency of the air conditioning system is also different for each vehicle.

However, it has been tested that driving with windows down with the AC off can consume more fuel than driving with windows up and AC on. This is because the wind resistance of the windows open at highway speed consumes more horsepower than the air conditioning consumes.

Keep in mind, on some vehicles, the AC can run when the heater is on to remove moisture. This is especially true for defroster for most all vehicles. The AC is needed to chill the air to remove moisture. Then it is heated before blowing against the windshield. If the air wasn't dehumidified before heating, then the air would deposit moisture on the cold windshield instead of removing it.

I hope that answers your question.