RMS power, Root Mean Square, is a measurement used in electric and nautical fields to define the power of stereos or fishfinders.

For a stereo system equipped with an amplifier or for a loudspeaker, the power expressed in Watts RMS is normally equal to half the musical power and one eighth (1/8) of the peak power. So a peak power of 80 watts equals an RMS power of 10 watts.
When the type of power measurement is not indicated, the peak power is normally meant.

Even in an echosounder the RMS power is equal to 1/8 of the peak power.