One of the potential issues that (maybe I missed and it was discussed) is that an alternator does not put out it's maximum or even optimum output when the engine is at idle speed. Have you ever noticed your lights get a bit brighter at night when you go from a slow idle to higher RPM if you have, say, your A/C going, your radio on, your interior lights on etc.? It's not that your alternator isn't working properly, it just isn't keeping up with the load at that point. Although it isn't very scientific, as a rule of thumb, your alternator isn't keeping up (and you'll end up with a dead battery) if you notice your lights get brighter when you turn something off while your engine is at operating speeds.
Older vehicles had generators and voltage regulators. Newer vehicles have alternators with voltage regulators built in. So, with the alternator keeping up, it will still be putting out it's optimum output and will not drop off of it's top voltage output when you turn on additional lights because it basically has 'reserve' output that is being limited by it's voltage regulator. Vin's link shows this by using a car stereo amp. They are telling you that when you are listening to music and your subwoofers hit (they draw the most) your lights will dim at that time because of the sudden draw that exceeds your alternator's output. But, since that huge draw is not constant, a large capacitor will handle that sudden draw and prevent your lights from dimming - and everything else experiencing a voltage drop. For car stereo, the general rule is to use 1 farad of capacitor for every 1000 watts of power. In my Jeep, with 3000 watts RMS of amp power, the lights will pretty well go out when a bass note hits if I don't have my capacitors in line.
Anyway, lighting doesn't have spike draws like stereo amps do. They have a constant draw. So, for lights, a large capacitor is useless. You need to have enough alternator power to recharge your battery, run your engine, and keep all accessory items powered. To be safe, I say you can use Bob's formula to see what power draw you need. Then I'd recommend you have an alternator that has an additional 30% or so additional capacity.