If you want to compare theoretical capacity then, depending on the efficiency of the solar cells, a 150 Watt panel might be 3' x 5' or +/- 15 sq ft. However as Sparrowhawk points out, that capacity is max and only available for so many hours in a day, fewer in winter and also in northern latitudes.
Theoretically fuel can be cheap
. One cell discussed in another thread, the E Foy uses methanol for fuel. Even very pure, reagent grade methanol in small quantities is only a few dollars per gallon. But the E Foy requires you to use some kind of fuel cartridge to fit in their unit that was about 10 times more expensive. Kind of like ink jet printers. The ink is only a few cents but the cartridge to fit your printer might be $20-$30.
The Hydromax fuel, who knows. Their web site is pretty vague and ambiguous. The fuel is listed as: a salty solution and fresh water
and apple acid. Does not say what kind of salt
so could be regular NaCl (table salt) or some exotic metallic salt
. A solution and fresh water
? But a solution could be aqueous IE salt in water so you add more water??? Then apple acid? There are a number of acids in various quantities in an apple including citric acid, malic acid, ascorbic acid, linoleic acid, linoleic acid, etc.
Bottom line for me, fuel cells are a very expensive way to add extra DC power to a boat. They may be clean, reliable, convenient, compact and more but not cheap
If your main criteria is the convenience and cost doesn't matter a fuel cell could be great. Before buying
I would check not just the cost but also availability of the fuel. If it is some exotic blend in a special cartridge I would make really certain the company will be around for a long time to supply the fuel or I would make sure I could mix up my own.