I run 9" chart plotter, 9" fish finder, VHF, CD, NMEA 2k network and wash down. A grp. 27 battery couldn't keep up a full 8 hour day with 30 amp output and an hour of run time
Keep in mind that you only get 10 amps for every 1 hour of run time above 1500 rpm
Multiply your run time (above 1500)...