A winglet is provided for retrofitting to a wind turbine. Aerodynamic and centrifugal forces for winglets having a range of configurations including winglet height, taper ratio, twist, and cant angle are modeled, wherein the winglet height, taper ratio, twist, and cant angle are used to define a grid in a Vector Lattice. An increase in a coefficient of power C.sub.p of each winglet design when applied to a predetermined main blade of the wind turbine can be determined. A winglet configuration can then be selected wherein the coefficient of power C.sub.p of the main blade and winglet is at least 2% greater than the coefficient of power C.sub.p of the main blade alone, and wherein a ratio of normal aerodynamic force generated by the winglet to centrifugal force generated by the winglet during rotation at a nominal rated speed is in a range between 0.75 and 2.