A capacitor is useful if your alternator already has the capability to fully drive your system. If not, I wouldn't spend the money. Get a better battery up front and upgrade the factory power/ground wiring under the hood and you should be fine.

Its not wattage that determines wire gauge, or whether caps are needed, its amperage. So take the fuse ratings of all your amps, add them up, and see what you get.

And, ill be nice about this: Its CAPacitors.