Hm... perhaps 15 watts could be enough. My first panel was a 7.5 watt panel that I kept on the deck and, as Gary predicted, sat on (not stepped on, sat on). So that's god advice there, as is the advice about regularly cleaning the connectors. Hadn't thought to use WD-40, though.
Still, last year I did a few 12+ hour singlehanded trips, so the tiller pilot tended to see some heavy use. I was happy to have the 30 W panel basically supplying its power for a good portion of the day and not have to worry about starting the motor (my batteries are also dual-use).
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27