the correct rating is contiuious sine wave power, into an appropriate load using 1KHz for music and 400Hz for speech. using load and scope set level to just below clipping, read RMS voltage. Hence E (volts,RMS) squared over Z (ohms), this gives you the power output. depending on the accuracy you want, a VOM will provide about 5%, precision RMS voltmeters will give 1% accuracys. there are resistors made for the loads.
