How can I tell if I need an 80amp or 100 amp alternator? Is there a big difference between them?

I was having slow starts this weekend and had AutoZone check & charge my battery. The techsaid it was good and then checked the alternator which he said had low voltage. I bought a new alternator off the AutoZone website, but it had a chaoice betweenan 80amp ora 100amp. Could it be anything other than the alternator?

After they charged the battery, I have not had a problem starting as of yet. It has been 3 days so far.

Thanks for any help!

 

I forgot: I have an automatic98 S70 with 156,000+ miles.

 

It should say on the back of the Alt what the amperage is.
Also have the battery terminal checked as well.

 

If you got the higher amps it souldnt matter as the car wont use what it doesnt need.