It Can be a Load On Both
During engine start , alternator helps to charge battery , capacitor , and power the amp
when engine is switched of ,
then , Capacitor will be charged by battery @ 12v ( weakly charged )
because capacitor ( requires 14.4v to charge up rapidly , depends on capacitor manufacturing material )
Capacitor , only helps Voltage ( no to drop sudden , or instantaneously )
its doesnt help for current
as Power = V x I
voltage drops , power drops
Less current flow , less power
Inductor : helps for current , not voltage
and hooking Capacitor in your sound System makes
one of the type of FILTER CIRCUIT