Energy efficiency is becoming more and more important in the HPC field; high-end processors are quickly evolving towards more advanced power-saving and power-monitoring technologies. On the other hand, low-power processors, designed for the mobile market, attract interest in the HPC area for their increasing computing capabilities, competitive pricing and low power consumption. In this work we study energy and computing performances of a Tegra K1 mobile processor using an HPC Lattice Boltzmann application as a benchmark. We run this application on the ARM Cortex-A15 CPU and on the GK20A GPU, both available in this processor. Our analysis uses time-accurate measurements, obtained by a simple custom-developed current monitor. We discuss several energy and performance metrics, interesting per se and also in view of a prospective use of these processors in a HPC context.