Enova has developed two systems: a charge depleting plug-in system and a charge sustaining system. The latter uses smaller batteries and operates similarly to most current parallel hybrid systems. The plug-in system used on the Napa bus uses a larger battery system that can be charged from the grid and run down during the duty cycle of the bus. The plug-in system can get up to a ninety percent reduction in emissions and one hundred percent reduction in fuel consumption based on the duty cycle.
[Source: Enova Systems]