If PL is the power supplied to the load and PT is the total power supplied by the voltage source, then power transfer efficiency given by
The voltage source E supplies power to both the load resistance RL and to the internal resistance Ri = (Rg + R)
The maximum value of η is unity when RL= ∞ and has a value of 0.5 when RL= Ri. It means under maximum power transfer conditions, the power transfer efficiency is only 50%.
