Microbial fuel cells (MFCs) are a high-priority sustainable-energy technology due to their ability to deliver usable current through an external circuit while simultaneously biodegrading organic molecules in waste streams or natural environments. For maximum power output, the external and internal resistances should be matched. Alternatively, sacrificing power for operation at high current under a low external resistance could be a route to achieving high rates of substrate degradation. Regardless of the application, reliable assessment of performance during long-term operation is essential. To this end, a polarization test is usually a key measurement, providing polarization curves (voltage versus current density) and power density curves (power density versus current density).