Any head unit will produce the power they rate it. But that power is usually measured at an unrealistic voltage with one specific test tone on a bench somewhere. Now in a realistic environment, they won't come close to that power in an RMS rating. This is the same in alot of companies who still rate their amplifier power in "peak" instead of RMS. There is soon to be an industry standard rating system, but who knows how enforced it will be.

I know Alpine states their RMS power of their head units right in their product descriptions. It gives you a more realistic amount of something like 27w/channel.

 

CEA 2006 I think is the new rating. Many amps and head units are already using it.

http://www.ce.org/Standards/Standard_Details.asp?id=556