In my opinion, a 15 Watt panel is mostly "cosmetic". In use, it will rarely deliver more than 7 Watts due to the inefficiencies of solar panels and the design of that particular panel and the included controller. It MIGHT, just barely, maintain the charge on lead-acid batteries when they are not in use. (It overcomes the self-discharge that lead-acid batteries have.) But, it won't do anything more.
If you are adding solar panels, I assume they would be at least 100 Watts each. They should not be connected to the little 15 Watt panel and you should look into directing their output into an MPPT controller, rather than a PWM controller if you want the best charging from them.
- Jack