A computer-based method is provided of forecasting total electric vehicle state of charge and total electric vehicle energy storage capacity in a parking area having parking bays with respective electrical vehicle charging and discharging facilities. Electric vehicles occupying the bays can provide temporary energy storage capacity to an energy grid. The forecasting method includes: forecasting future occupation of the bays by electric vehicles based on expected arrival times and expected stay durations; obtaining expected charge and discharge capabilities of the vehicles; forecasting a total state of charge and total energy storage capacity in the parking area; recording actual arrivals and departures; and adjusting the forecast of future occupation and the forecast of total state of charge and total energy storage capacity in the parking area.