High-performance computing (HPC) aims at devel-oping models and simulations for applications in numerous sci-entific fields. Yet, the energy consumption of these HPC facilitiescurrently limits their size and performance, and consequently thesize of the tackled problems. The complexity of the HPC softwarestacks and their various optimizations makes it difficult to finelyunderstand the energy consumption of scientific applications. Tohighlight this difficulty on a concrete use-case, we perform anenergy and power analysis of a software stack for the simulationof frequency-domain electromagnetic wave propagation. Thissolver stack combines a high order finite element discretizationframework of the system of three-dimensional frequency-domainMaxwell equations with an algebraic hybrid iterative-directsparse linear solver. This analysis is conducted on the KNL-based PRACE-PCP system. Our results illustrate the difficultyin predicting how to trade energy and runtime.