Microbial fuel cells (MFCs) harness the natural metabolisms of microbes to produce electrical power from almost any kind of organic matter. In addition to the low power densities (about 1mW for a 1-liter reactor), MFCs are presently built with expensive membrane and electrodes. The payback time of MFCs is therefore very long (evaluated to 25000 years for our lab prototype). Progresses in designing low-cost MFCs are necessary before conceiving large scale energy production.