Operating photovoltaic (PV) modules are frequently shaded by nearby structures, vegetation, droppings, etc., and this reduces the effective incident solar radiation received by the modules. Shading also reduces the power output of PV modules and, under certain conditions, causes the formation of hotspots. In this study, a wide variety of partial shading scenarios were investigated to evaluate their effects on the output current, voltage and efficiencies, and hotspot formation in mono-crystalline and poly-crystalline PV modules operating under the ambient conditions experienced at Nsukka, Nigeria. Sixteen shading cases were considered, including 20%, 40%, 60% and 80% of the modules' surface areas shaded parallel to the long sides, parallel to the short sides, diagonally and randomly. Test ambient conditions, module outputs and surface thermal patterns were simultaneously monitored using a digital solarimeter, multimeter and infrared thermal imager, respectively. The outputs of the modules reduced to almost zero when as little as 40% of the module surfaces were shaded, with the reductions in performance being more severe in the mono-crystalline modules than in the poly-crystalline modules. The infrared thermography revealed the thermal patterns under the different shading conditions and showed that the random shading of the modules was the most likely to result in hotspots.