About Watt Ratings

The most accurate and meaningful rating method for amplifier output power is the so-called RMS rating, which would read like this for a 150-watt amp:
"150 watts per channel, both channels driven, 20-20kHz, at 'x' percent distortion". (Note: for solid state amps, distortion should be below 0.5% -- distortion for tube amps will typically run somewhat higher).

This rating system specifies that the amp will produce a given output across the audible frequency range, at or below a certain distortion level, with both channels driven (driving only one channel is a much less stringent standard). Some amps do not have good power supplies, and thus will produce less power at the frequency extremes (which means that deep bass frequencies will typically not be well reproduced).

The other rating standard is "peak power", sometimes called dynamic or transient power. This rating reflects the power output capability of the amp under brief loads, such as orchestral peaks (rather than steady-state power demands). Most good solid state amps will have dynamic or peak power output that ranges from 50% to 100% higher than the RMS output. While the dynamic output is useful to know, the RMS rating is much more important.

In Europe and Asia, amplifier power ratings are more frequently quoted as dynamic or transient/peak power. The problem of amplifier power becomes a bit more complicated when rating tube amplifiers, since tube amps are essentially voltage sources, rather than current sources. Tube amps may clip somewhat more gracefully than solid state amps as they approach their maximum output. Tube amps may also produce higher total amounts of distortion than solid state amps, but the distortion occurs in different harmonic orders that are (supposedly) less objectionable to human hearing.

Sdcampbell, nice post. I might add that the impedence load is also an important part of the rating. "150 watts per channel, both channels driven, 20-20kHz, at 'x' percent distortion into 8 ohms" would be a more complete description (add RMS, which averages the power across the sine wave, into it and away we go). In the old days, during the power rating wars, "peak power" has also meant the power rating at the top or peak of the sine wave. RMS power is about 70% of that. "Peak to peak" power (and I wonder if that isn't part of the PMPO abbreviation) is double peak -- i.e., it counts the absolute value of the power to the peak in both the positive and negative sine wave cycles. So, peak to peak may be 200w and peak is half that at 100w and RMS is about 70% of that at 70w -- all are accurate descriptions of the same amp, but they measure power in different ways.

Getting back to the impedence load, a 70w amp into 8 ohms might deliver 140w into 4 ohms (if it has the power supply and design capacity to do so) and 280w into 2 ohms, which sometimes is the impedence for very low bass. So, it is perfectly legitimate to describe the same high quality amp as 70 watts RMS into 8 ohms and 600 watts peak to peak into 2 ohms. Add in all of Sdcampbell's other mentioned caveats (e.g., distortion level differences, applicable bandwidth) and you could claim even more power from that amp. Good thing this hobby's so simple ;-)

Thanks for the responses. Coming back to the dealer's comments on Audio companies not reporting the correct watt ratings - as a user, how can I verify this?

For e.g. One multichannel amp (with helding the name - don't want to get into any sort of trouble) is rated at 375 watts @ 4ohms but a review of the amp revealed that it only capable of driving 300 Watts at 4 ohms!

I had an "EQ Booster" in my first car. It was the coolest thing. My pimpled teenage face was all grins. If I recall correctly, it had a PMPO rating. I think it was Peak Music Power Output and was a claimed "200watts." Hehe, yeah right.

Of course, in typical teenage excess, I needed more. It wasnt long before I had a shiny new Carver M-240 (120x2 RMS) and a pair of Babb 6x9s. Holy cow, pubescent heaven, and arguably the loudest midrange blaster I have ever heard.

Anywho, RMS is the number to look for and it usually has some additional measurements along with it to better define the test. Such as the sine wave(at 1khz)or sine wave sweep(20 to 20khz) and distortion in percentages at rated power.

But I digress the stats are truly just the tip of the defining iceberg. To make an analogy, lets say you wear "large" Shirts.....but that in no way means you will like all "large Shirts." Color, tailor, fabric, design, etc., etc. will play a more important role. You'll probably try that shirt on to see if you like it. I try to do the same with Hi-Fi purchases.