Question: how many watts does it take to charge a 12 volt battery?
Answer: I.e. Assuming You have a typical full size auto battery, it is about 50 amp hours. To Calculate how much time You'll need to charge the battery with a 15 Watt solar charger You'll need: Calculate the Ampere per hour of the charger: 15 Watts /12 Volts = 1,25 Amperes.