Power generated by microbial fuel cells (MFC) is at a maximum when the rate of charge transfer to/from the current limiting electrode is at a maximum; this power is called the maximum sustainable power. This power is used to determine what external devices can be attached to the fuel cell and the maximum power the cell can deliver continuously. Predicting the external resistance that is associated with the maximum sustainable power in MFC is difficult because the operator has limited influence on the main factor that controls power generation: the rate of charge transfer at the current limiting electrode. To avoid these difficulties, and to utilize the maximum power of MFC, an empirical procedure is designed to predict the maximum sustainable power. This procedure characterizes the best possible way to operate a fuel cell. The procedure is simple, the fuel cell can be characterized within an hour. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 10/30/2005-11/4/2005).