In this application, it’s recommended you select at least an 80W panel, or, alternatively, the largest panel that will fit on the available roof of your rig. Skimping on panel size in the first instance may seem wise based on cost analysis, however it is something you will likely soon come to regret. As mentioned, a 75W panel can generate approximately 4-4.5A at maximum, or more useful to us, around 10 to 20Ah/day. That said, the more panels you have in your system, the greater energy can be generated in the same amount of time.
If you are using a compressor-type refrigerator in your rig (particularly anything larger than the smallest of portable units), you will likely need to increase this recommendation to at least 3, ideally 4 75-85W panels if your goal is to maintain long-term self-sufficiency on the road. Below this recommendation and you will find the system begins to struggle during times of cloud or high ambient temperature. Also remember that beyond refrigerators there is a number of things which will draw power from the system, including TVs, pumps, power inverters, and even your lighting.