Recent data have shown that it could be profitable on some control markets to use Plug-in Hybrid Electric Vehicles (PHEVs) as control power resources. The concept where battery driven vehicles such as PHEVs provide ancillary service to the grid is commonly referred to as Vehicle to Grid (V2G). As each PHEV has a limited capacity, it is necessary to have a control system that aggregates a large number of vehicles. This paper investigates what is required in order to design such a system. The result is presented as reference architectures and contains propositions of important components, processes and information needs. It is shown that a PHEV can provide control power in two different ways that in turn generate several different system concepts. A mathematical model is presented that allows comparison between the different solutions.