How to calculate power consumption (amp hours) for a car stereo?

Very hard to do with a stereo system, depends how efficient everything is.

The only sure way would be us an amp meter but that would only give you a guide, you might have to use something like a data logger and play you typical music at your typical volume for a set time and then calculate the average current draw per hour.

Again using an amp meter would only give you a peak reading not an average reading unless it has an average function.

With car audio power out put don't think what they say is is what it is.

Also as the battery decreases in charge so does the voltage, when the voltage goes down the current goes up.

We had a Yamaha 6.1 AV amp rated at 100 watts true RMS for each of the six channels and the advertised power consumption was 710 watts, we then brought a Cambridge Audio 7.1 AV amp that is rated at 100 watts for each of it's 7 channels but the power consumption is 1400 watts....some of the reviews out of the UK have tested the Cambridge at 198 watts RMS per channel.

So if you used the RMS power out put as a guide the Cambridge would be miles out.

This all depends on how loud you have your stereo. I installed a 4 x 50 watts stereo in our kombi 6 months ago, been working fine and loud and just the other my son turned it up all the way and the inline fuse blew. I found that there was only a 2amp fuse it there where as it should 16amp. So therefore all this the stereo had been drawing less than 2 amps.
Dave

easy answer is "who knows" because music is very transient in its power requirements, and is so dependent on the volume (gain) required from the amps.

Yes you can always use the fuse ratings on your amp to give you a worst case scenario, but it all depends on the music being played - ie lots of bass up loud etc etc...

just install the biggest batt you can fit and "play it by ear".

Connect an ammeter in series to the unit, run it for an hour at a level you are comfortable with, taking readings every 10 minutes or so. Get an average. Presto! An average amp hour usage.
Good luck!