How many Farads in a capacitor do i need for my new subwoofer setup?

I have a car with a custom audio set up and i'm about to change the subwoofer setup to run 500watts RMS and 1000watts peak but i'm currently running 230rms and the idle speed on my car already drops when the sub hits at idle, i was looking into capacitors and i was wondering how many Farads do i need in a capacitor to keep this from happening when i get my new subwoofer.