The trolling motor is a 30lb thrust motor. I use it atleast twice a day to go out to my Tilapia cage to feed them. I fish with it as well, so it gets 15minutes of run time daily atleast.
I'll take your word for it and go with the 15 watt panel. I'd rather spend a bit more now than go cheap and get something worthless.
Another ? Will the 15watt panel from HF stop charge when battery is full? Or is that what the volt meter is for?

Hard work beats talent when talent doesn't work hard