Not sure what part of Idaho you're in. Definitely more sun down south, but up in the Coeur d'Alene area, I'm pretty sure 15 watts would do little. A little snow followed by an extended cold snap, and that panel will deliver next to zero. I guess if you have good permanent southern exposure... I kind of like Pianotuna's recommendation of a larger panel secured vertically.
My trailer has two 75 watt panels on the roof that provide plenty of camping power and enough winter maintenance for my two 6v batteries with only a couple hours of direct sunlight daily max (if I'm very lucky). When the snow gets deep and sticks around, I plug in the shore power.
