The method includes connecting a MOSFET-type transistor to the photovoltaic installation; applying, to the transistor, a signal of a control voltage (Vgs) that crosses a linear regime range (φlin) of the transistor, between two critical voltages including a saturation voltage (Vgs(sat)) and a threshold voltage (Vgs(th)), and measuring the current and the voltage of the photovoltaic installation while the linear regime range of the transistor is being crossed. The control voltage signal (Vgs) of the transistor is generated from a digital control signal. The transistor initially being in short-circuit (φcc) or open-circuit (φco) regime, a command is issued for a first, rapid variation (BT1) in the control voltage (Vgs) in the direction of the linear regime range of the transistor, then a second, slow variation (BT2) in the control voltage (Vgs) crossing the linear regime range of the transistor, the transition between the first and the second variation being discontinuous.