Beyond a critical disorder, two-dimensional (2D) superconductors become insulating. In this Superconductor-Insulator Transition (SIT), the nature of the insulator is still controversial. Here, we present an extensive experimental study on insulating Nb_{x}Si_{1-x} close to the SIT, as well as corresponding numerical simulations of the electrical conductivity. At low temperatures, we show that electronic transport is activated and dominated by charging energies. The sample thickness variation results in a large spread of activation temperatures, fine-tuned via disorder. We show numerically and experimentally that the localization length varies exponentially with thickness. At the lowest temperatures, overactivated behavior is observed in the vicinity of the SIT and the increase in the activation energy can be attributed to the superconducting gap. We derive a relation between the increase in activation energy and the temperature below which overactivated behavior is observed. This relation is verified by many different quasi-2D systems.
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