I have found, at least up north here, that with the amount of daylight, the amount of parasitic draw, 15 watts is about the minimum to keep the batts up to snuff consistently.
Remember, just the losses in amperage due to the cord, controller and connections, you'll lose a good portion of that 7 watts.
In my case, my 15 watter puts out about 700mA (over and above the parasitic load) for maybe 6 - 7 hours a day...on a SUNNY day. Less than that during morning and evening, and of course nothing at night.
The parasitic drain, after moving the propane detector to a storage switched location is between 150 and 200mA.
It takes one sunny day to make up for one cloudy day. Your results may vary in sunny SoCal, but the rest of us need to be aware of our latitude and average sunny days. Also, I found the propane tank wasn't a good location for me. Shaded by the trailer part of the day and you really need to angle the panel for the best capture of rays at 90* to the panel for as much of the day as possible while in storage.
"If the women don't find you handsome, they should at least find you handy." - Red Green