In this chapter, it is demonstrated that the photovoltaic cell which forms the basis of one class of solar power system is essentially a semiconductor junction diode formed from doped crystalline silicon. The diode action is developed here in electrical engineering terms. Consequently, when it is illuminated with light, the photovoltaic mechanism can readily be explained by viewing the device as immersed in an electromagnetic wave which has the effect of modifying the quiescent state of the diode, thus causing a photo-current to flow. The basic photodiode equation, which forms the 'bedrock' of solar PV module and array design and development, is key to array operation and control. It can thus be evolved, as the chapter demonstrates, from the laws of engineering electromagnetism supported by fundamental thermodynamics. For regular solar modules and arrays, employing identical cells throughout their structures, it is shown that the cell's theoretical model is formally applicable to the whole module or even the entire array. Measured I-V characteristics are available from manufacturers for photovoltaic diode cells, and it is observed that the efficient and effective incorporation of such cells into modules and arrays necessitates the matching of theoretical parameters to experimental data. There is a plethora of mathematical techniques to do this, but here, we have concentrated on the Newton tangent method which cogently illustrates the process. The chapter concludes with a brief examination of array power collection and efficiency from a theoretical perspective. The need for power level control, and panel directional tracking of the sun, in order to maximise these important parameters is given some consideration in the penultimate section, while in the final section, the implications of technology advances on array efficiency are perused.