I have heard of solar panels
which are 2x more efficient than mono-crystaline (about 30% efficient) but they cost 30x as much. Might not be practical for most of us, but if you could double the power without any extra space needed...
I have two 115W panels ($200 each) and I only get 10 watts out of both of them during a heavy downpour when the sky full of dark clouds.
Fortunately my computer only uses 7 watts, so it makes enough power to continue to surf the web the whole time.
I think a catamaran
would be the best boat to have lots of solar panels.