Hello there - Did you ever find an answer to this question? My old boat had a Promariner plug in charger and old slip had shore power. My new 2008 Seafarer has no charger installed nor new slip has power - so I am also looking at solar for dockside music. So far, i believe a 100W, 5Amp kit would keep up w/ a stereo (non amp) power draw. Or that's what I looking at...
I have no shore power too and boat is on a lip for almost year around
Ex owner gave me two 170W Ecoworthy flexible solar panels to install but i have not installed them yet.
I bought a Victron solar charger and the cables and as soon it's fresher i will install them and go over the whole electrical system and plan to replace the house battery with the biggest LiFePo i can fit.
Take the measures to figure out what size you can fit and take the biggest ones, the price difference is usually not a lot.
If you should replace the flooded with LiFePo you will have about the double of usable Ah than wet ones with half of the weight.
When i am on my boat i can hear music with a Fusion Amp and the MFD's on for at least 4 hours and the hard wired house and starboard 100Ah batteries still crank the starboard engine.
Regarding the charger shown in the Amazon photo above, i have the same but without display and i consider it as a cheap, not very good charger and suggest to spend a bit more and get a serious solar charger. The Victron i have has Bluetooth and a app who show battery status, charging state and amps what is always nice to know.
Chris