Car Stereo head unit power is VERY deceiving! Head units all have HUGE power ratings on the Boxes or right on the front of the units! The highest head unit power I have ever actually tested was 13 watts (at a half way decent THD)… So even if the head unit says 35Watts or 40 watts per channel on it don’t EVER expect for it to REALLY BE more than 15! Just cant happen!

REAL amplifiers use LARGE power supplies in them with transformers to STEP UP the voltage So that large amounts of power can be made! Head units just don’t have room in them For transformers or large power supplies!!!! So NEVER expect more than 15watts per Speaker out of head units! Some of the more reputable head units might LIE on the boxes but If you read the small print in the spec sheets in the owners manual they might admit to the 15watt figure!

This picture shows a typical car amplifier.
The amplifier can generate much more power per channel than the car head unit.
Why can't a head unit generate this kind of power???
In the picture above you will see a large power transformer (coil of wire) in he middle of the amplifier (labeled 4)... This transformer converts the 12 volts from the car battery into a much higher voltage (in this case +36 volts and -36volts)? Since the car head unit doesn't have a transformer to boost the voltage (not enough room in it) the head unit can only limp along trying to make a lot of power with the measly 12 volts from your car. Most high power head units use a bridged output (also called floating ground or push pull), head units use a little 12 volt IC chip that makes both speaker wires to each speaker work together with one pushing while the other pulls (so to speak) just like a real amplifier does when you bridge it. But unlike a real amplifier, most car head units cannot be un bridged. But even still, having no transformer in the head unit limits it severely and your output will be around 12 to 13 watts pr speaker.
Don't believe the high power LIES.