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 the large transformers that are needed for large power supplies!!!! So don't expect more than 15 watts per Speaker out of head units! Some of the more reputable head units might print high power ratings on the boxes but If you read the small print in the spec sheets in the owners manual they might admit to the 15 watt figure!

Most high power head units use a bridged output (also called floating ground or push pull), head units use an 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.  

How can they print these specs if they aren't true? Well, for starters, car audio is not regulated like home audio. The Federal Trade Commission several years ago required home audio manufacturers to print accurate power ratings based on a standard testing procedure. Car audio did not yet exist (much), so this requirement was not worded to include car audio. As a result, les scrupulous manufacturers will print the peak power rating, as opposed to the RMS. A peak power rating (usually called "maximum" because it sounds better) is a measure of what the amplifier can do for just a very short time. RMS (often called "continuous", because it sounds better) is a more true measurement. This is how much power an amplifier can continue to put out for longer periods. Accurate peak and RMS measurements give a good view of an amplifiers capability (the peak measurement is required to produce the heart pounding bass, which requires much more power than the continuous music, which needs RMS power). However, there is no standard for measuring peak power among manufacturers, so it can't be trusted for most equipment.