Various countries and communities are defining strategic energy plans driven by concerns related to climate change and security of energy supply. The long time horizon inherent to strategic energy planning requires uncertainty to be accounted for. Uncertainty classification consists in defining the type of uncertainty involved and quantifying it. It is needed as input for uncertainty and sensitivity analyses, and optimization under uncertainty applications. In this work we define a methodology for uncertainty classification for a typical strategic energy planning problem. As an example, the methodology is applied to some representative parameters.