Before you start thinking about the electrical system you need to realize how much energy you need, and think whether or not it’s worth putting up 2 solar panels or having two batteries.
In our case our big energy consumer will be the refrigerator, which will consume about 20 Amperes per day, then it will be basically the lighting, some electric kitchen appliance that we use and carry the mobile phones, photographic machine, computer …
So we think that a battery of 100 Amperes (attention that should only use up to 50% of the capacities of the batteries, so we can only use 50 Amperes) was fine.
Our battery will charge in two ways, through the solar panel (100w), or through the movement of the van (a reley attached to the main battery). As Portugal has many hours of sun a year, and we intend to make km with the van, we think that it will suffice the 100w of panel and of 100Amp of storage.
For countries with less sun it would be complicated to have the system that we have with only a 100w (solar) / 100amp (battery).