Car Stereo deck power is very deceiving! Many head units 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 massive power supplies in them with toroidal 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 the unit's built-in amplifier circuitry. 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! Alpine's V-Drive head units are stated to put out 60 watts per channel, using a direct 10AWG power line directly from the car battery's positive terminal, but once you read the fine print, you'll see that the actual power output is a maximum of 26 watts per channel continuous. This is still one of the most powerful head units on the market... at 26 watts.
Most high power stereos use a bridged output (also called floating ground or push pull), 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. Unlike a real amplifier however, most car stereos cannot be un-bridged. Even still, having no transformer in the head unit limits it severely and your output will be around 12 to 13 watts per 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, less 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 amplifier's 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.