Plug-in hybrid electric vehicles (PHEVs.) can become valuable resources for an electric power system by providing vehicle to grid (V2G) services, such as energy storage and ancillary services. We use a unit commitment model of the Texas power system to simulate system operations with different-sized PHEV fleets that do and do not provide V2G services, to estimate the value of those services. We demonstrate that a PHEV fleet can provide benefits to the system, mainly through the provision of ancillary services, reducing the need to reserve conventional generator capacity. Moreover, our analysis shows that PHEV owners are made better off by providing V2G services and we demonstrate that these benefits can reduce the time it takes to recover the higher upfront capital cost of a PHEV when compared to other vehicle types.