This paper focuses on the estimation of series resistance changes for flat silicone PV panels or arrays during operation, without moving the operating point far away from the Maximum Power Point. The method is based on the measurement of the slope of the IV curve at a current level that differs from the short-circuit current with a fixed value, regardless of irradiance conditions. The method causes less power loss and power ripple due to measurements and it is easily integrated in a hill-climbing MPPT algorithm.