14.5*.2 = 29W ?? actually 14.5*.2 = 2.9W. Have you measured what current is actually drawn by original mains charger. Anything over 2.9W is loss from charger circuit. Probably not much.
My garage door opener recently failed due to nearby lightning strike which killed controller board (so repairer said). I asked a few questions while unit was opened up. I found that controller and motor both ran on 24V DC.
So if OPs unit runs on 12V throughout it should be simple to run from...
