Partial shadowing of photovoltaic systems can overproportionally reduce the energy yield and lead to early ageing and failure of the shadowed cells. Large area shadows are relatively easy to detect due to the eminent power reduction and decrease of fill factor. However, small area partial shadows in a larger system do not have a very obvious effect on the output power or fill factor of the PV array and can remain undetected, leading to failure. In this paper a method for detecting small area partial shadows, based on equivalent thermal voltage, is presented. A simplified expression of the equivalent thermal voltage is proposed, which increases the robustness against measurement errors and model limitations at low irradiation conditions. Experimental results confirm the high sensitivity of the method even to a relatively small area shadow, while showing very good robustness against increase in series resistance.