This paper presents a parameter estimation technique for the circuit of a photovoltaic module. The proposed method is based on unscented Kalman filter for the joint estimation of the state variables and the parameters involved in the model equations, using external measurements only. A case study is presented where the obtained estimation errors remain lower than 3% in all cases. Additionally, three formulations of the Kalman filter are compared in terms of convergence and accuracy. This technique may contribute to the optimal operation of photovoltaic power plants and the maximization of the investor revenue.