what size alternator do I need?

By the draw you place on it. If your car comes with a 70 amp altenator then assume that the car will probally use about 60 amps 90 percent of the time. So if you put in an amp that has a max draw of say 50 amps, you could probally safely assume that for normal listening its gonna draw up to 40, anything past that is, in all likelyness, only gonna be very brief so you dont have to be concerened about it. so right now you are up to needing at least 100 amp altenator to keep up with your system and car. You can go bigger and assume that the car will draw 70 amps and your amp is gonna draw 50 and buy a 120 amp altenator, but it probally isnt nessasary. I hope this helps somedouble-secret reverse-osmosis speaker-cone-induced high-level interference distortion, Its a killer

... and for quick reference check the " Power and Ground" page for a chart with amplifier power VS current draw.Support the12volt.com

Thanks DYohn. That page explains it pretty clearly. I think I might be ok with my alternator I have now. Its 136 amp alternator. I am running 2 kicker zr 360's which both have 2 40 amp fuses in them. One sony amp that has 2 30 amp fuses in them. I believe Im running between 1000-1200 watts.

The quick math for determining the true wattage of your amps is take the fuse rating multiply that 12.5 (your avg car battery suppling power) and then write that number down and mutily then by .4 (b/c most amps only run at 60% efficiency) so therefore 140x12.5=1750 1750x.4=700 1750-700=1050 true wattage Maybe if you are continously running @14.4 you will achieve 1200watts.

Like the old saying goes, "Friends dont let friends buy Sony"Tim-May!