Abstract: The performance of a photovoltaic array implemented in the Block A Rooftop Solar Plant at National Institute of Solar Energy (NISE) which installed last 20 years ago is analyzed. This plant consists of 80 monocrystalline PV modules with a MPP power of 35 W under STC. This plant is divided into 4 parallel strings with each of 20 modules connected in series as string. The mismatch losses on different array configurations are analyzed firstly using measured IV characteristics for each module using a solar analyzer device, and secondly using MATLAB SIMULINK model. Infrared (IR) thermography testing is used for hot-spot [detection], cell line checking is used for interconnect integrity and electroluminescence (EL) testing is used for [detecting] degradation of some modules.