Do The amp hour ratings on all car batteries really matter as much if your car alternator is working well? Ah is the capacity but if you have reliable charging do it still matter as much? Cause its going to keep charging the car battery so as long as your not drawling over 50% or 60% you should be fine.
The main thing your battery has to do it start the car, and act as a 'buffer' for the alternator.
Starting the car is by far the highest load on the battery. Most car batteries are rated in Cold Cranking Amps (CCA) to determine how well they will turn the starter when it is cold out.
The battery stores power, accessories draw power, the alternator feeds power to the battery. The battery keeps a steady supply of power. The alternator's power isn't always 'clean', but the output from the battery is. When you use something like the horn, there is a spike in power draw, and the alternator is not able to handle spikes well, so the battery picks up the slack. At idle the alternator is not putting out full power, so the battery may need to pick up the slack. This will drain the battery slightly, but the alternator will recharge it once the vehicle is at higher RPM again.
Any time you use accessories with the engine off, you are drawing from the battery. Common items are electric locks, alarms, automatic lights, etc.
Overall, the amp hour (ah) rating does not matter much as long as you do not use accessories with the engine off. The main thing you need to worry about is your alternator. You can watch the voltage to determine if the alternator is able to keep up.
engine off - ~12.5-12.8 engine on - ~13.8-14.2
If you turn on your headlights, fog light, radio, AC, window defroster, then check the voltage and it is lower than 13.8, it means your alternator is having a hard time keeping up. You can try reving the engine to about 2000 RPM to see if it will keep up better while you are moving.
Another note on voltage dropping at idle. I've had several cars with underdrive pulleys. They spin the accessories slower to reduce load on the engine. I would always see my headlights dim at idle indicating that the voltage was dropping. Everything was fine once I started moving. Overall, I never had issues.