I think you mean 15 watts.
I can't remember the exact #'s but a 15 watt panel in full sun for 8 hours was about 1.5 to 2.5 amp/hrs per day.I think a group 27 battery is 180 amp/hrs.
So this is the way i look at it,
150$ solar for 10 minutes of light (approx) if its sunny per day.
100$ new battery 2 days of lights ,furnace and music or hook 2 together for 200$ and get 5 nights of power.