iadd-up mo lahat ang power ratings ng lahat ng electrical consumer ng sasakyan mo. refer ka na lang sa google sites how to estimate such things.

quick formula is the ohm's law: P = IV where, P-watts, I- amperes, V-voltage.

then check mo rating ng alternator....for longer life ng alternator 60-80% percent or even lower ang loading nya parati. huwag mo i-100%.