 The last step in calculating the Betz efficiency "C"p is to divide the calculated power extracted from the flow by a reference power value.The Betz analysis uses for its power reference, reasonably, the power of air upstream moving at "V"1 contained in a cylinder with the cross-sectional area "S" of the rotor.The Betz limit has no dependence on the geometry of the wind extraction system, therefore S may take any form provided that the flow travels from the entrance to the control volume to the exit, and the control volume has uniform entry and exit velocities.