You go to the store and look on the back of an amplifier or stereo system amp and you see 100 watts. I used to think that was 100 watts to the load, but lately I've come to reconsider this assumption. An amplifier has an efficiency of between 25 and 75 percent. So, in the worst case an amp rated 100 watts actually supplies 25 watts to the load and in the best case supplies 75 watts to the load. My question is, how can the consumer know the actual power to the load?