1. The problem statement, all variables and given/known data The power supply for a stereo amp develops an output of 22 volts at 2.8 amps. If the overall efficiency of the power supply is 78%, what is the input current that it will draw from a 117 volt source. 2. Relevant equations P = EI 3. The attempt at a solution Eo = 22 V Io = 2.8 Amps Po = 22 * 2.8 = 61.6 watts Pi = 61.6 / .78 (efficiency) = 79.0 watts Ei = 117 V Ii = 117 / 79.0 = 1.48 amps Basically is this right? I thought what you put in always has to be bigger then what you get out? if so then how can 1.48 amps in = 2.8 amps out?