For designing a high-efficiency RF-DC conversion circuit, simulations and trial and error usually occupy a large part of the work. Such design method needs much time. The important part of design is to design an input circuit. The efficiency of a conversion circuit mainly depends on input circuit. However, it is difficult to design the input circuits. This study takes notice of the voltage when the DC load resistance is open. The open DC load voltage is an important quantity for design of the high-efficiency RF-DC conversion circuit. We show by experiments and simulations that the efficiency of an RF-DC conversion circuit tends to increase with the open DC load voltage. This relation can make it easier to design RF-DC conversion circuit.