A system for recovering and utilizing vapor from a source of vapor has a vapor holder for storing a quantity of vapor from the source of vapor. Also included is a condenser coupled to the vapor holder for receiving and condensing at least partially, vapor from the vapor holder. The system also has an engine and a generator driven by the engine for generating electrical power. The engine has an engine intake coupled to the condenser and an exhaust outlet. This engine is powered at least partially, by output from the condensing apparatus. The system also has a fuel adjustment apparatus and a fuel sensor apparatus. The fuel adjustment apparatus has a control input and is coupled between the engine and the condensing apparatus for adjusting fuel concentration into the engine intake in response to a signal on the control input. The fuel sensor apparatus is coupled to the engine intake (a) for sensing concentration of at least some constituents of vapor at the engine intake, and (b) for applying a signal to the control input of the fuel adjustment apparatus corresponding thereto. The system also includes an exhaust sensor apparatus coupled to the exhaust outlet for providing an exhaust signal signifying concentration of at least some constituents of the exhaust at the exhaust outlet.