Global energy is in a major shift from energy using fossil sources to renewable energy using more sustainable energy sources. The wind is the clean and inexhaustible one of renewable energy resources that is available in most parts of the world. In order for wind power generation to meet more ambitious targets around the world, it is necessary to understand all the physics behind process of wind generation. The most interesting of this process is the power coefficient which involves the real characteristics of the wind turbine as a function of the generated power. Up to now, the power coefficient does not yet have a general form but are fitted as mathematical functions from manufacturer data measured from popular wind turbines in the world. In this paper, we develop an analytical physics-based method to estimate the power coefficient general for various wind generation systems of horizontal-axis wind turbines (HAWT). Instead of 12 parameters in semi-experimental exponential formula , our power coefficient uses only 5 parameters with their physical meanings. By varying these five parameters we also obtain the power coefficients fitted from experimental data.