whats the rule for how big of an alternator you need for a system?

[rookanga 10+ year member]

I have a trail blazer right now, im going to get a subaru forester soon, and change a lot of things to, including sub box, batteries in the back and another amp, right now I have a fi sp4 and a sundown saz2000, im going to get another saz2000, that should hit nicely, dont know if I should go anymore then that thou, but how big of an alternator should I get and how big of an alternator should I really get for everything

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[IonRL205 10+ year member]

I'd say at least a 270 amp just so if you decide for more power you already have a good alternator.

I was going to get a 270 amp and 4 batteries when I upgrade to a 4k watt system next spring.

 

[JoeK 10+ year member]

I have a trail blazer right now, im going to get a subaru forester soon, and change a lot of things to, including sub box, batteries in the back and another amp, right now I have a fi sp4 and a sundown saz2000, im going to get another saz2000, that should hit nicely, dont know if I should go anymore then that thou, but how big of an alternator should I get and how big of an alternator should I really get for everything

How is that SP4? A 270amp looks like it should be enough with a couple batts

 

[CAT MAN 5,000+ posts]

the rule is as big as you can get

 

[rookanga 10+ year member]

tell you the truth I dont really know how good the sp4 is, im only pushing 2000 watts to a 2500 rms sub and the box is to small for the sub, but I heard if you put a bigger alternator in, it makes the enigine work harder, will that affect the mpg of the vechicle?