Sorry guys, I just returned.
I intended to replace my reply asking about the "10% extra" with a more direct reply...
The 10% means an upgrade may not be required.
VIZ - that 10%
overhead is for a vehicle idling in peak-hour traffic in the cold dark rain. Hence the alternator outputs sufficiently - plus 10% reserve - at near idle speeds with headlights on, wipers on slow, heater fan and rear demister on, and normal audio, stop lights and indicators. And it will still recharge the battery after several short stops and longer than usual cranking times on an old battery.
Ergo, headlights & wipers & demister off and you have an extra 30A available.
Increase from idle speed and you have even more available.
Assume the 700W RMS amp is at 1/4 volume hence maybe 500W input. That's under 50A which the system may well handle.
My point is that I often see those types of "10% extra" or "minimum additional cost" statements. But 10% extra to what? [And if they theoretically needed a 67A alternator, they wouldn't build it, they'd get the next existing size (maybe 75A) or maybe a 90A or 120A alternator if they are cheaper (thru mass production, or with fitting and bracket costs etc).]
As I see it, manufacturer's do not want vehicles to fail in the rainy dark cold peak-hour scenario I mentioned, so that is their baseline - NOT the "normal" daily cruise scenario.
How best to design? Well apart from being prepared to add the extra amp/load requirement (at 100% power all the time, or 25% half the time, etc?), it's by observation.
IOW a voltmeter. And preferably before the addition so you get familiar with how often the alternator is
under-voltage yet the battery reliably cranks next time, or the battery is fully charged by the journey's end.
(Refer elsewhere to 12.7V and under being battery discharge, up to 14.4V being a normal charge, and over 14.4V long-term leading to battery destruction.)
Some have upgraded from 45A alternators to 200A alternators only to be worse off. (IE - many OEM alternators output more current at lowish RPM than do high-output alternators.)
And they only found out when their vehicle failed to crank later...
Now, if they had a voltmeter...!
As to later posts, forget caps. They do nothing to increase the power output of alternators - they can only store charge. And that charge is far exceeded by a battery. (Even a 1.2AH battery holds more charge than a BIG multi-Farad capacitor.)
In general, the only use for a cap is in BIG audio systems (2-3kW and above) where adjacent AGM batteries are used, and that's ONLY to protect the AGM battery (ie, take off some of its surge requirements).
When caps are used for filtering, a battery is generally far superior, plus the battery has the advantage of a much larger capacity for better sag ride-thru.
If the extra battery (instead of the cap) is 7AH or 15AH or larger, it can be an emergency cranker. How many caps can do that?
Of course a 2nd battery must have some form of isolation when not in use. That might be an automated system (alternator controlled relay aka "the UIBI", or a voltage sensing or "smart" isolator), or even an IGN or switch controlled relay, or mechanical links.
Only when that is not practical or possible might a cap be considered - eg, a cap across headlights to minimise dips - but then a faulty cap will act the same as a faulty parallel battery.
As to caps or batteries and alternator loading, forget it.
Some say they reduce alternator loading though probably most say they increase alternator loading.
In fact the former is the truest (extra or bigger caps & batteries REDUCE the demands on the alternator), but it should be irrelevant anyhow since alternators are self-limiting wrt to output current, and only poor designs will fail under normal situations (including jump starts!).
If audio buffs reckon they increase loading, then they should logically support having smaller batteries, and of course never an AGM!
Of course they might only argue against a cap since they have no charge-current limit per se whereas batteries will have a limit (even if it exceeds their specification).
But audio buffs that are in to (peak) SPL competitions will usually tell you NOT to have a cap, and similarly not to use AGMs, or not
electrically adjacent AGMs).
The other advantage of the voltmeter - your super-duper alternator has worn brushes and is dropping output. You'll know about it before it's too late. (It will also pre-alert you to a failing battery.)
There are lots of other considerations like do you need peak voltage (14.2-14.4V) or how often do you want the battery to discharge and recharge - and to what level.
I have only skimmed some basics.
Oh - and BTW - the BIG-3!