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

Question

I want to calculate actual power consumption of my car stereo, for the purposes of planning a dual battery kit.

1. The head unit says "4 x 52W". I've only connected 2 speakers to it. Let's assume "52W" is the peak.

2. The standalone amp says "4 x 50W RMS".

Is it simple enough to calculate the power consumption? I've used a few web calculators that indicate 300W equates to 14.5A.

My next calculation would be 14.5 x 4 hours x 2 days = 116AH (amp hours) used. This seems massive to me ... and would add considerable expensive to my dual battery project.

Have I got it right? Am I better off going to an auto-elec and getting them to physically measure the power usage?

Thanks in advance for your help!

Answer 1

Unfortunately there's no easy way to calculate it, because your system's current requirements will vary wildly depending on what it's playing and how loudly.  Under normal use, most amplifiers are only using a fraction of their available power, except during musical peaks.  Furthermore, amplifiers aren't 100% efficient, so you can't easily use the power output to determine the power input.  

Possibly the easiest way would be to use a "clamp" type multimeter with a DC current setting, and measure the current flow through the battery cable when the system is in use.