I use a 30 pound pushpower trolley (probably the smallest) on my inflatable dinghy. The consumption is about 230 watt. I use two 80 amp batteries. One in the dinghy, the other on board the ship for charging. One charge last for 4 hours use. For charging I use a 2,5 KW gas-generator.
If were not that I also need to use the generator for a whole lot of other charging purposes, like charging the batteries for use with refrigerator, TV, computers etc. I would probably spend more gasoline, make more noyce charging the batteries for the dinghy than i would using a small 5 -6 HP two-stroke for propulsion.
I believe you need to make an input/output calculation of the charge/use of current. It is the charging capacity that will define the distance of "the point of no return".
Use your head, ram the wall till it falls.