Despite the increasing popularity of computable general equilibrium (CGE) models in energy-economy-environment analyses, Polish data still provide a modest disaggregation of energy-related sectors. Hence, CGE modellers need to disaggregate corresponding products and industries on their own – to not only obtain more detailed insights, but also avoid the problem of an “aggregation bias”. The aim of this paper is to test for such a bias in Poland’s case using a small open economy, CGE model called GEMPOL, with an in-house split of energy sectors. Three alternative versions of the model are calibrated and solved. The first version includes energy sectors in their original breakdown. The second version includes their in-house split, with particular values of Armington elasticities derived directly from “parent” sectors. In the third version, Armington elasticities are increased in order to reflect the higher degree of international competition for smaller sub-products. Through a simulation shock, imposed under comparative-statics mode, an exogenous energy efficiency improvement is modelled. Finally, the results obtained from all the variants of the model are compared. It turns out that the simulation results produced by both aggregations and all three specifications of the model are similar from the macroeconomic perspective, but they vary significantly between different aggregations at the sectoral level.