How will I know the Power Efficiency of a load?
A Thevenin equivalent is a voltage source with a resistor in series like this:



For power to flow you need a load which is Rl in this schematic. The load at which maximum power is transferred is when the load is the complex conjugate of the thevenin output impedance. If there are no reactive components, it is when the resistances of Rth and Rl are the same.

Power dissipated in load = Vth^2 * ( Rl / (Rth * Rl)^2 )

These equations come courtesy of Bucknell University. Read more here