50A, 6AWG only goes total out and back distance of 15ft with < 3% voltage drop. That means if distance from alternator to house battery is 7.5ft ir more, 4AWG. Unless the Redarc output voltage is settable to say 15v, then any drop more than 3% will be problematic charging batteries correctly. 7.5 ft is just over arms length, unless solar battery is in engine bay it is probably further. If not, then 6awg.