Electric power grids require that power generation and loads are in balance in order to keep the grid frequency constant. When there is an imbalance between the generation and load, the grid frequency will change. Grid frequency regulation is usually a service performed by powerplants that are controlled to vary their generation output up and down from a nominal value. Plug-in vehicles can provide a similar function by varying their charging rate based on locally-measured grid frequency. As plug-in vehicles reach large scale adoption, there is the potential for all frequency regulation to be provided through charging of plug-in vehicles.