Future fleets of vehicles may include electric vehicles (EVs) or hybrid electric vehicles (HEVs) because of potential fuel savings. Recent demonstration of diesel parallel hybrids in a delivery fleet led to fuel economy improvements, and hybrid bus demonstrations exhibited twice the fuel economy of the conventional bus. Fleet ownership may include management of a fleet of vehicles as small as 10 units and as large as hundreds or thousands. In addition to fuel savings, the newer extended range electric vehicles (EREVs) and pure EVs permit vehicle to grid (V2G) opportunities. These V2G opportunities may present additional revenue for fleets by providing ancillary services to local grid independent system operators (ISOs), provided that the burden of driving and V2G services do not accelerate the degradation of the battery systems in these vehicles. The subject of this study is to determine the financial risks associated with accelerated battery degradation in a V2G-enabled EREV fleet expected to perform ancillary service duty while charging in addition to the normal loads of drive cycle duty. We determine that battery cycle life during V2G duty is a critical parameter, which can determine whether or not the business model is viable. (C) 2011 Elsevier Ltd. All rights reserved.