The standard charger has an output of 4 amps at 12 VDC or 48 watts. Given inefficiencies in the AC/DC conversion in the charger the inverter required to run the charger , the amp draw from the house batteries is probable more like 4.5-5 amps. that said, 100 watts of solar panels should be enough to keep up with charging a torqeedo battery.

It would also be pretty easy to make up a charge limiter circuit so that you can charge directly from the house battery bank.

Based on observations from the year I have had the Torqeedo, I can say that the power required to push a rib-lite 310 with two people and gear in it one mile (round trip from the dinghy dock to my mooring) at 3 knots ranges from 60 to 80 watt hours, or approximately 12-16% of battery capacity on a Torqeedo 1003 with the 520 watt-hour battery. With the standard charger, which puts out 48 watts, recharge time is around 2 hours after a 1 mile run or about 9 amp-hours from the house bank. If you slow down to 2.5 knots the power draw drops by about 35-40%.