looks like RPIJG was pretty much right on the rating. according to my work's website (www.kragen.com
), it looks like the alternator is 110 amps for the zetec, and 105 for the SPI. vanace, you assumption as to how the charging system works is more accurate. everything pulls off the alternator, then goes to battery as backup. provided you have a good battery that's fully charged, it's not really the high loads that kill an alternator as much as the surges, and that's where a cap helps out. caps only discharge as fast as they charge if you ground them.
PS- on a total side note, never disconnect your battery to see if your alternator is OK. it worked on older cars, sure, but it really can screw with the electronics in newer ones.