power of an amplifier is determined by the voltage rails and the maximum amount of current its electronic components can handle for an instant without damage. This characterizes the ability of equipment to handle quickly changing power levels, as many audio signals have a highly dynamic nature. It always produces a higher value than the average power figure, however, and so has been tempting to use in advertising without context, making it look as though the amp has twice the power of competitors. Total system power Total system power is a term often used in audio electronics to rate the power of an audio system. Total system power refers to the total power consumption of the unit, rather than the power handling of the speakers or the power output of the amplifier. This can be viewed as a somewhat deceptive marketing ploy, as the total power consumption of the unit will of course be greater than any of its other power ratings, except for, perhaps, the peak power of the amplifier, which is essentially an exaggerated value anyway. Shelf stereos and surround sound receivers are often rated using total system power. One way to use total system power to get a more accurate estimate of power is to consider the amplifier class which would give an educated guess of the power output by considering the efficiency of the class. For example, class AB amplifiers can vary widely from 25% to 75% efficiency while Class D amps are much higher at 80% to 95%. An exceptionally efficient Class D amp, the ROHM BD5421efs, operates at 90% efficiency. In some cases, an audio device may be measured by the total system power of all its loudspeakers by adding all their peak power ratings. Many home theater in a box systems are rated this way. Often low-end home theater systems' power ratings are taken at a high level of harmonic distortion as well; as high as 10%, which would be noticeable. PMPO PMPO, which stands for Peak Music Power Output or Peak momentary performance output, is a much more dubious figure of merit, of interest more to advertising copy-writers than to consumers. The term PMPO has never been defined in any standard, but it is often taken to be the sum of some sort of peak power for each amplifier in a system. Different manufacturers use different definitions, so that the ratio of