Standard airfoils historically used for wind and hydrokinetic turbines have maximum lift coefficients around 1.3 at stall angles of attack, which is about 12°. At these conditions, the minimum flow velocities to generate electric power are about 7 m/s and 3 m/s for the wind turbine and the hydrokinetic turbine options, respectively. In this study, NACA4412-NACA6411 slat–airfoil arrangement was chosen for these two options in order to investigate the potential performance improvements. Aerodynamic performances of these options were both numerically and experimentally investigated. The 2D and 3D numerical analysis software were used and the optimum geometric and flow conditions leading to the maximum power coefficient were obtained. The maximum power coefficient was 0.506 at the tip velocity ratio of 5.5 at the optimum geometric parameters of h/c1 = 0.273, c1/c2 = 0.394, δ = 16°, and the angle of attack of 10°. The experimental work conducted in a towing tank gave the power coefficient to be 0.47 which is about %7 lower than the numerical results obtained. Hence, there is reasonable agreement between numerical end experimental values. It may be concluded that this design and optimization approach may be applied in the design of wind and hydrokinetic turbines for electrical power generation in lower wind and current velocities compared to standard airfoils.