You will not be disappointed with 120W even if mounted flat. With little care and 200 AH battery bank it is possible to stay without a generator on cloudy days. Reduce the laptop time.
Top guns have said what they had to, you should've listened though. 40ft cable - what if you need 41ft? And mysterious 20A controller. Too late for remorse. Now you have to figure out how to attach it to your roof. Included Z-brackets aren't much useful, L-brackets are preferable for flat mounting (and allow one-axle tilt). Depending on the roof material you might also have to find hardware other than included screws.
