When individual cells appear hotter, yielding a patchwork pattern like that shown in Fig. 5, the cause can usually be attributed to defective bypass diodes, internal short-circuits, or cell mismatch. Distinguishing between these faults may be accomplished via a simulation of the circuit. For example, more advanced students can use SPICE to develop a model of the solar module and study the effects of bypass diodes, parasitic resistances, and cell mismatch on the dark and illuminated I–V characteristics.10 For the introductory student, it is sufficient to note that a lack of uniformity among cells implies disparity in the current generation mechanism, which presents a challenge to Kirchhoff's junction rule. Since the least effective device will ultimately limit the current throughout the rest of the circuit, excess energy will be lost in the higher performing cells. Quality control in the manufacturing process is important for minimizing the impact of any defective cells.