Energy detection constitutes a popular sensing approach thanks to its relatively satisfactory performance at low complexity requirements. Its efficiency can be practically enhanced by employing diversity schemes which are also capable of providing adequate mitigation of multipath fading effects. Based on this, the present work is devoted to the analysis of energy detection based spectrum sensing over generalized multipath fading channels using square law combining. Unlike the traditional evaluation based on the receiver operating characteristic (ROC) curves, the present analysis is based on the area under ROC curve (AUC), which is a particularly accurate performance measure that is used widely in natural sciences and engineering. To this end, novel closed-form expressions are firstly derived for the conventional AUC over the generalized κ - μ fading channels. These results are subsequently employed for deriving closed-form expressions for the case of square law combining. It is shown that the corresponding performance is, as expected, highly dependent upon the severity of fading and is improved substantially as the number of branches increase. In this context, it is also shown that using up to five branches ensures rather acceptable performance even at non-high signal-to-noise ratio values. Furthermore, the offered results have a relatively convenient algebraic representation and can be useful in analyses relating to cognitive radio and RADAR systems.