I have a fan with three different speeds. On the bottom it says that it uses 43 watts. Does this value go to the slowest speed or does it always use the same amount of electricity.

 

The 43 Watts is probably the maximum consumption. That would cater to the safety instinct gene.

 

That is likely for the highest speed and the power goes down as the speed goes down.
Buy a Kill A Watt or similar meter to measure the actual power if interested.