I have an 8500 w genny but I went with a switch plate on my electrical box, a lot cheaper than one of those sub boxes plus I can run whatever I want, I've have the central air on running other stuff and never had a problem. Cost a lot less to do the switch plate, cost me $500 for the switch and have the electrician set up the plug outside and he made me a 15 foot cord. I had $1,000 plus quotes from other electricians to do it the other way.