How can I figure my alternator needs?

Hi everyone, what rules/formulas does a person use to figure out their HO alternator needs?  I have searched and seen a bunch of "what size alt should I run" posts and there are always helpful answers but I the only wisdom I have read is "buy the biggest you can afford" and I am sure there is more to it than that answer.  Do I just take the load of the aftermarket equipment and add it to the OEM alternator rating?  example, adding Orion 8004 & 2500D amplifers (three 4-ga 100A fused input leads, 300A draw total) to a 2003 Chevy truck with stock 105A Delco alternator:  is the required amperage 300 + 105 = 405A???  Or is this thinking flawed?

TIA -

Mr. P. :)

ps moderators - can I suggest this information be included in the Big-3 Upgrade sticky?

What will dictate the size is simple. Build quality and warranty of the unit. Next you need to consider the mounting location. You can only get so much power into such a small space. After that consider how you will be using the system, balls to the wall, well for 400 amps of draw, you are going to need a second alternator or a bunch of batteries. Considering that music is full of peaks and valley's as far as current draw goes, the addage of buying as much as you can afford based on the quality you can afford is the correct answer.

Thanks man, I really appreciate your setting me straight.  At first the answer seemed that straightforward to me, but then I let myself get confused by a lot of other "conventional wisdom" like 'dude a 200-amp alternator works just fine' etc.  If you want the horses you've gotta feed them the hay!

Mr. P. :)

pretty much the way to figure it is to add up the total current draw of all the aftermarket equipment and add that number to the specs of the OEM alt and take that total for minimum output for the new alternator.  for example:  if you stock alt is putting out 90 amps, and you have 3 amps each with 50 amps of current draw, then take 3 x 50 = 150 + 90 = 240 amps...so buy a 250 amp alternator (i havent heard of a 240 amp alt so i rounded it up).  its always better to go a little over than under.  also get a couple extra batts for reserve power.  in your case you would be correct but i dont think anyone makes a 400 amp alt so 2 200 amp alts will have to be installed.  aftermarket alt companies make brackets for multi-alt applications and just need a longer belt is all.  also upgrade the "big 3" for sure.  check out the big 3 upgrade post in the car audio section.

Don't forget to account for the car's own draw itself. MOST cars will draw about 35-40 amps too. It doesn't sound like much compared to your 400+, but it makes a big difference in your alternator(s) choice.

About 95% of electrical systems in cars will only take a 115% load of the alternator before it really causes damage to the electrical. Your alternators (note plural) will have to total 460 amps or more to be able to run the system safely without risking electrical.

Batteries provide a buffer, but honestly it is more like a band-aid. If you are pulling 405 amps out of the system, then your factory battery (for example) will give you about 1.5 - 2 seconds of energy before it ends up getting deep cycled (internal voltage dropping below 9v; not total voltage). Once it get's deep cycled, the battery can only recharge to about 75% of it's original design so now you have only a 1-1.5 second buffer)

Moving to say 2 Stinger 2400s for example will give you about 9-11 seconds of juice before D-C and about 6-8 after. They'll also set you back about $850.

Dual 240 alternators will set you back about $1000-1200. So you spend an extra buck50 or so, but your system will be secure and safe to run. One company based in Florida claims to have a 550 amp alternator, but the price is so outrageous $2500 just for the alternator, no bracket, pulley, fan etc that it isn't worth it unless you have money to burn and/or are VERY tight on space.