I have a 25w on a 150g. It seems to be ok. Just make sure you have a low water flow going into your sterilizer to ensure longer contact, But also you dont want it to low, or it may burn up.
I also read somewhere before that its not till you get 18w(or somewhere around there) you kill certain types of bacterial and algee. But I do agree anything over 25w is just overkill. 18-25 I think may work best for you.