Re: Gen/Tran
I agree the gen trans is good for a retro fit, however most contractor in my area don't use it as the circuit breakers are only 15 A. More commonly I see CH or Sq D small load centers fed from two side by side two pole CB's- both mfgs make listed interlock kits. What you end up with is a dedicated "emergency" panel, for new construction you can lay out the emergency circuits. With a Gen Trans, if existing, you get what circuits you can.
Another popular panel is made by Eylander Electric in Everett WA, near by to me. It is typically a 200 amp panel with two side by side interlocked breakers, listed by UL.