Needed Current Calulations

[Zermelo 10+ year member]

I found this Excel sheet which calculates what size alternator and power wires you need as well how to set the gain:

http://www.datafilehost.com/download.php?file=6d26c621

Are the calculations accurate? I am asking because the calculation seem to be based on too little variables, for example if you type in a wattage, it seems to spit out a needed amperage, but wouldn't this also depend on the type of amp used? Aren't class D amps suppose to be more efficient than ABs? So wouldn't that make a difference in the calculation?

Also, I was reading this (http://www.caraudiomag.com/specialfeatures/0207cae_audio_system_power/index.html) on the Car Audio and Electronics site which states as a general rule to see how much current an amp will need you divide the rms wattage by the voltage (somewhere around 13.8 for most cars). This seems too simple to me again because of the variance in amplifiers.

I just wanted to know if either of these guides give a good rule of thumb for electrical system upgrades.

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[Innovative:Cory 10+ year member]

google ohm's laws.... and have a fun read..