Don't buy one of these !!
My house has one. When I bought the place the inspector and I noticed an electrician had a report posted where he replaced a "burned up panel". Both of us wondered why but could not find one thing wrong with the wiring in the house or incoming power, it was actually quite good in fact. Later when hooking up my generator I powered down my main as a matter of practice but noticed that this "interlock" did not fully prevent both PUD power and GEN power from feeding the circuits at the same time. Then I recalled when we looked at the house the previous owner had a brand spanking new generator. So power came back on and let the smoke out of his generator and electrical panel.
So unless you want to risk your generator, electrical panel and maybe A LOT more, I would not install this. It may not always be me that is at home when the power goes out and my wife or kids likely will not be able to make sure the breakers are in the correct positions. I will install a Square D (or other commercial brand) transfer switch at some point.