Air conditioning is a fairly constant load, while open windows (and the additional drag) increases with speed. So when driving around town, windows open can be more economical, while at the motorway air conditioning will be more economical.
Air conditioning will also use cooling and reheating in case of high humidity (condensation will occur within the system, drier air will be sent to the passenger space). So the air conditioning might be on more then you expect, using a bit more fuel.
That said: after 1 or 2 years it should be covered under warranty.