watts (say a 100 watt globe)divided by volts(use 13volts because the engine is running)equals the amperage you need ,in this case 100 divided by 13=7.69 amps,,tally up all the watts, like tail light brake light ,indicators,imagine everything is on and so long as the demand dosent out strip supply itll be ok ,,so long as you have a buffer of about 10% ,these are an old 70s brushless alternator and are lucky to charge at idle these days even with modern rec/regs.,they just handle a 65watt halogen head light,im sure someone has done a beefier stator winding somewhere.,watts is the output of the electrical device,amps is the current it draws.