This paper introduces a new method for the preliminary design of power controllers (PCs) in the electric power grid. The method, which is denoted the ideal phase-shifter (IPS) method, utilizes the concept of the power controller plane where the active power of the PC line is plotted versus the difference in voltage angle between the PC terminals. The power controller plane makes it possible to graphically visualize the working area of a PC in a power grid and thus determine the grid situations which are dimensioning for the PC. The IPS method offers the possibility of plotting the grid characteristics in the power controller plane which are unbiased with respect to the reactive properties of the PC. This makes the method suitable for comparison and preliminary design of PCs of different types and with different characteristics by simple geometrical considerations. In this process, the IPS method uses the power-flow control method for deriving the PC characteristics. This paper includes an application example of the method where it is used for dimensioning of two different PCs in a 26-bus test system.