A much more serious and reliable measure is watts rms. Rms stands for root mean square. The rms power is found by averaging the instantaneous power output over a long period of time.
To get an idea of the relationship between PMPO watts and watts rms, consider the following numbers advertised for some current loudspeakers. These models have been selected at random, and inclusion in or exclusion from this list is neither a recommendation nor a criticism.
AC power, which includes audio, is rated in watts rms, which is equivalent in heating value to DC watts. This is the honest method. A given direct coupled amplifier (transformers are rare in non-distribution equipment) can obviously only swing its output from ground to VCC at best. This limits a simple car amplifier to something like a 12 volt swing peak to peak.
For example, an ideal (100% efficient) amplifier with a 12 volt peak-to-peak supply can drive a signal with an amplitude of 6V. Into an 8 ohm loudspeaker this would deliver
With a differential (push-pull) output the swing is doubled so the power increases by a factor of four. Therefore, with a 14.4 V supply, which is the highest voltage usually available from a fresh car battery, the maximum possible output into a four ohm speaker is about 14.4 x 14.4 x 0.707 / 4R or about 46 watts rms.
"Music Power" is a somewhat arbitrary multiplier based on the fact that amplifiers cannot provide full power for long, but can produce high power in short bursts. This allows them to reproduce normal audio, which has a high peak-to-average ratio.
To get more power, high-end equipment uses a DC to DC converter to generate a higher supply voltage (at the expense of drawing more current from the battery).
The true power output of an amplifier can be estimated by examining the DC input. Linear amplifiers tend to be about 60% efficient at best. An amplifier labelled "500W PMPO" but fitted with a 5 amp fuse can therefore deliver an average power of 5A x 14.4V x 60%, or about 43 watts.