Try taking a look at some local shops in your area. My buddy owns an auto shop and upgrades any alt to 160amp for a very minimal amount or can build bigger sizes for fairly cheap. So just a suggestion check out some local stuff and see if you can't just get an upgrade for much less.
Just my opionion.... DONT EVER buy from a Audio Dealer in a local shop, without first pricing around. The audioshops by me, the only one i could find that had bigger alt. was Music in Motion.. and they wanted 1100$ for it, but thats installed to...for no charge....they said....LMAO and thats only a 200A i can get a 300A for 450$
Like I suggested 250amp because remember that your vehicle came with 90 to 100 amp for factory equipment and that 153a is only what the amp needs so you still need 60 to 110 amps extra depending on the extra equipment you have or might add. Plus this leaves room for upgrades. Hope that helps.
"for example if your system is 1600 watts and 12 volts it would be 1840/12= 153 amps"
This may be true for full volume test tones, but I don't think there are many of us that have listening habits quite so extreme.
Even bass-heavy music at full volume will require about 1/3 test tone power. So realistically - rms power for that amp, at full volume playing music is going to be around 50A.
It's this principle (crest factor) that explains why thousands of people can (and do) run 1000w+ systems in their civics and corollas for years and never have an electrical system problem.
I'm all for beefing up an electrical system when it's needed, but 250A for an efficient 1500w amp is way overkill IMO.
(assuming an average car with average stock electronics) -- If it were me -- I'd look for something around 150A and expect it to be enough for the HF and a good sized a/b to run the front stage. However, I don't have very high demand listening habits - if you do, a 200A alt might be worth the extra $.