Determination and monitoring of series resistance of a photovoltaic device or system from voltage factors

Computer-implemented methods, devices and computer programs for determination and monitoring of series resistance of a PV device or system. One of the methods comprises: computing a voltage factor, VF, of the PV device/system as a ratio between: an open circuit voltage, Voc, and a voltage at a maximum power generation, V_MPP; computing a partial derivative of VF with respect to a current at the maximum power generation, I_MPP; computing a first ratio between: a product of a predetermined constant, n, associated with the device/system times a thermal voltage, V_TH, and I_MPP; computing a second ratio between V_MPP and Voc; and computing a value for a series resistance, Rs, at least based on: a product of the partial derivative of VF and Voc; and a product of the first ratio and the second ratio.