it's quite simple there is 2 ways you easily know 1st,if you have amp meter connect to batter + terminal it will show you or connect to generator hot wire (+ wire )
2nd if you don't have meter then you need 2 things these are 1st,paper 2nd,pencil like this
head light 90wats /100watts + 90watts /100watts = 180wats/200watts
if you have hid
hid 3.7a+3.7a at 12v,14v = 7.4a @12,14 volts
separately calculate amp and watts
in amper also note the volt
and start with your heads lights both point high or low , if you have hid then check Amp of both the blaster and write on paper
you need to note down every parts like fog light,parking lights,indicator,back lights,break light,indoor lamp,sound system power,and if you have audio amplifier then check its AMP ,car battery charge @ 8 to 12 amp and car e.c.u take max 0.5,2amp