Quote:
Originally Posted by jamesqf
1 HP = 746 watts. I see typical stereos advertising 200 watts/channel, so if you crank the volume to the max (goodbye, eardrums!) you could theoretically lose about half a horsepower.
Of course in the real world, you'll be using less than that even at full volume, because the sound level will vary...

At this time, NO indash car head unit has a DCDC power supply providing rail voltage to its amplifier IC. (some units, such as a discontinued Panasonic model and some specific Alpine and Kenwood units could be fitted with an outboard switching DCDC unit to double rail voltage)
What this means is, for all practical purposes 100% of all indash car head units you can buy are working with 012 (or 14.4 under ideal conditions) volts and the vast majority use BTL configured output transitors with a 0.5/1 bias.
14.4 volts into 4 ohms is 51.84 watts *peak* theoretical power per channel, if there are no losses anywhere else in the circuit. Which means RMS sine wave average output will be roughly half that, again under ideal circumstances. So you have 21 watts times 4 output when all 4 channels are rockin' full boogie 100% of the time, or 84 watts. Figure the head unit's other items (illuminated display, DSP, mechanical cd transport, etc) will use another 20 watts or so, and the whole shebang isn't going to be 100% efficient since BTL A/B type amplifiers  even chip amps like are typical in this application  are at best 75% efficient. So you're around 140 watts absolute maximum continuous draw under the most extreme theoretical conditions at full volume.
I really doubt anyone drives around playing sine waves at 100% volume all the time... so figure the radio at a reasonably loud volume is going to use less than 40 watts average. Not a concern worth even thinking about.
Sadly, no law prohibits manufacturers from making up ridiculous wattage claims for their products except specifically home hifi equipment. That is why $15 PC speakers can claim to be 1000 watts, and car stereos can claim to output ridiculous amounts of power. They arrive at those numbers using whatever funny math they want to justify it with, (for example, you can see above in my math how Sony arrives at "52w x 4" from their head units... 14.4x14.4 divided by 4 ohms equals 51.84 watts... rounded up is 52)