Here is an excerpt from a well regarded amplifier manufacturer. The first are the specifications (in detail) about one of the amplifiers that they manufacture. The second is a power FAQ explaing/selling about the way these amplifiers work and the testing that goes into them. Note the RMS vs. Peak vs. Avg levels.
Configuration: Mono Input:
RCA (Ref 8) or True balance XLR (Ref 8b)
Power output (see Power FAQ)
Power/Load 8 ohm 4 ohm 2 ohm
(20 msec hold time) 288W 576W 1152W
Avg. Power per Cycle 144W 288W 576W
RMS Power 100W 100W 100W
RMS power is the maximum continuous power of the power supply and it is constant. Avg. and peak power provide the required instantaneous power boost.
THD+N < 0.05% from 20 Hz to 20 kHz, 8 ohm load, 10 W.
Damping factor > 4000 at any audio frequencies, output (source) impedance is near zero for the entire frequency response
Input impedance: 47K ohm
Gain: 26 db
SNR = 100 db at 100W
Cardas connector & patented rhodium over copper binding post
Chassis is completely made of high grade brush and anodized aluminum to reduce audio resonance
Dimension: 6.5" x 10.5" x 1.75"
Worldwide AC voltage (84VAC to 264VAC). No need for AC regulator if the AC voltage falls within the specified range.
Weight: 3 lb
Reference 8 uses a 100W SMPS and the power output spec. reflects the limitation of the SMPS.
Q: XXXXXX's amplifier is so small and light. Does it provide adequate power?
A: Don't be deceived by the XXXXXX's amplifier's size. Our technology utilizes high-performance switch-mode power supplies (SMPS).
As opposed to traditional linear power supplies, which are always large and bulky, SMPS offers what at first looks like a
contradiction: smaller size and higher efficiency. More significantly, SMPS operates at much higher frequencies -- thousands of
times faster than linear power supplies operating at 50/60Hz. SMPS responds faster, thus providing the instantaneous power
high-performance music reproduction requires. Even a very small SMPS is several times more capable than a 50/60Hz linear power
supply. SMPS also offers sophisticated over-current and short circuit protection in addition to meeting industry safety and
reliability standards. XXXXX's cool-running SMPS provides an unequalled regulated output without the 100/120 Hz ripple voltage
found in linear power supplies, even when huge cans of filter capacitors are employed, as is normally the case with high-end amplifiers.
Q: What are Peak and RMS (Root Mean Square) power ratings? What's the effect on audio output?
A: RMS power is the continuous power an amplifier is capable of outputting over long periods. Peak power is the short-term power an
amplifier outputs when faced with sudden, high-energy signals. Another important measurement is the sine wave's Average Power Per Cycle.
Vpeak = Peak output voltage of amplifier
R = Speaker load in Ohm
Peak Power = (Vpeak)2 / R
RMS Power = Power supply's RMS power
Max Power Per Cycle = (Vpeak)2 / 2R
Headroom refers to an amplifier's ability to go beyond its rated average power (RMS or continuous power) for a short time, in the
recreation, for example, of loud, fast-rising audio signals. In order to achieve ample headroom, i.e., the ability to produce loud
peak levels without distorting, an amplifier must have a stiff power supply with a good amount of reserve energy. The ability of the
power supply to quickly recharge its capacitors is critical. An explosive low-frequency sound, e.g., at 50Hz, consists of an attack
followed by a series of rapidly decaying lobes of a 50Hz sine wave. An amplifier with 140W peak power with 20ms hold-time is capable
of providing 140W power to the 50Hz musical note (full cycle of 50 Hz is 20ms). For example, a series of explosive sounds seem to occur
in close succession, yet their base notes are far enough apart (1 second = 1000 ms) for the amplifier to provide the required peak
power. For typical home listening, more than 20W of power is rarely consumed. Even so, when demand exceeds 20W, a good amplifier should
provide RMS power and high-peak power with consistently low distortion. XXXXX's amplifier is capable of generating peak power at less
than 0.05% THD+N).
Q: Is XXXXXX's 300W amplifier less or more powerful than a linear solid state or tube amplifier rated for 300W?
A: In terms of reproducing music's dynamics, the more power the better. The commonly accepted measurement rates the output when the
amplifier clips at 1% THD. What is clipping? Clipping occurs in a linear amplifier when its output signal tries to exceed the limits
of its power supply voltage. In a linear amplifier using transistors and a bulky transformer/rectifier, the storage capacitor is recharged
once every 8.33 milliseconds, with its voltage only slightly above the maximum output voltage, a situation in which the amp could quite
easily clip. Simply put, the amp has very little headroom. With tube amplifiers, the supply voltage is very high, typically 300-600V.
Even though tube amplifiers have higher overall distortion, subjectively, they sound more powerful. XXXXXX's amplifier is a switching
design with ample headroom. It will not clip at its rated 300W. Even at this extraordinarily high output level, it will sound subjectively
better than a 300W linear amplifier.