Hey, I was just wondering if there's any rule of thumb or some sort of math to see how many batteries you need.

For example:

If u have a 150A alt and a 170A draw. Will you be fine with just one batt or do you need another. This is just a theoretical scenario

Do you have any option to add a larger alternator? 

If you could do that, then add one larger battery and call it a day.

I was just wondering if there was a calculation for how much amps a battery could make up for. General number is OK as I know every battery is diffrent

For daily listening

As far as i have always heard the common rule for amperage draw is Voltage X Amperage = Watts.  So for example you have 150A alternator and they have about a 40% reserve so thats about 60 amp's take that away from 150 and you get 90amps. So 90 x 14.4 (most common alternator output in volts) is about 1296 watts. The batteries Ah or amperage an hour falls into play as well but not until i think it hits resting voltage. I could be wrong but if you have a higher draw you'll also see less power to put to your system as well. Hope this helps and gives you a better understanding as well.

As far as i have always heard the common rule for amperage draw is Voltage X Amperage = Watts.  So for example you have 150A alternator and they have about a 40% reserve so thats about 60 amp's take that away from 150 and you get 90amps. So 90 x 14.4 (most common alternator output in volts) is about 1296 watts. The batteries Ah or amperage an hour falls into play as well but not until i think it hits resting voltage. I could be wrong but if you have a higher draw you'll also see less power to put to your system as well. Hope this helps and gives you a better understanding as well.

Ohm's Law is invaluable, but you must first understand how to apply it. How do you figure a 40% reserve for an alternator? There is no such thing. To figure out how much battery you will need, you must first determine how much amperage you are trying to supply. So here we go: Total Wattage/ Idle Voltage= Current draw. That is for system power ONLY. You must still factor any power the vehicle's electrical uses as well. AGM batteries usually specify the wattage amount they are able to support. I would opt for a larger alternator, and a good sized AGM battery like bassaholic said and go from there.