Anything you bolt to the house is going to have a fair degree of inaccuracy built in. Anemometer’s want to be in steady air flow and your house will be causing all sort of air disturbances.
A case in point is my old University. It was the tallest buildiing in the surrounding area; a tower block on top of a hill. The uni had several wind turbines installed on the roof – none worked as the air flow was too disturbed to be steady enough to be useful.
Relistically, the Beaufort scale aluded to above is going too be more accurate unless you put a anemometer on top of a large (4x height of house from memory) mast on your house…
Or take up sailing – That teaches you to be able to guage the wind speed by feel after a few years.