This research focuses on the stage of engineering a technical prototype of a composite wind power plant with a diffuser (WPPD). The area of a particular interest in the article is the mathematical aspect of the engineering stage. The article presents a theoretical study as well as experimental and practical data essential to obtain an effective shape of the diffuser. Here are also given engineering calculations and results of patent researches and field tests. The authors calculate the most rational design parameters capable of ensuring the maximum speed in the area of the blades. This leads to an increase in the generated electricity, since it depends on the speed cubed. The increase in speed is stipulated by the passage of the air flow through the narrow part of the diffuser into the area of expansion and is consistent with the Bernoulli equation. The differential equation relates the flow velocity to the cross-sectional area of the diffuser. It should be noted that its inner part is the surface of rotation of the generatrixes around the axis of the diffuser. The surface shape can be adjusted based on the obtained mathematical calculations. In the research, these curves are described in the form of polynomials of various degrees. After integrating the differential equation for each curve individually the best option is selected. Here is also given an example with a quadratic function, which was experimentally substantiated in earlier researches and is used to compare the effects received from different generators. The described technology, with account taken of the shape of the generator fairings, contributes to the further improvement of the WPPD. The article serves as the basis for engineering a technical prototype of a mobile composite WPPD.