Depends what you want to do with it but best performance is not gained by fitting to the roof. Ideally you want the panel at a 45 degree angle to the sun, therefore freestanding is more efficient and you can use a smaller one. My 50 watt panel has done us proud for the past 3 years with no EHU for any holidays. Sometimes let the little darlings watch a dvd at bed time and all lights in van are LED but at least one bunk light stays on all night!
Can't remember who I got mine from but it was ordered at NEC having done a lot of price research before going. The service was second to none with delivery prior to the promised two weeks quoted at the show. All I have to do on arrival is remove from bag, plug in to the socket and position. Regulator is on the back of the panel, and plug socket fits to battery terminals, came fully ready to go including padlock and chain. At bed time I turn for the morning sun and then move once more later in the day. Works well in all weather conditions even kept me going in the fog!