This paper describes an energy system model to investigate the energy demands and supplies of agents connected to an electricity network so as to calculate the possible future half hourly loads imposed on the network. The model predicts the change in scale and shape of current demand profile and forecasts potential changes until 2050, given a mixture of efficiency and supply technologies that might be installed over the coming decades. The model can simulate individual agents at a local distribution network operator level or aggregates of agents at regional or national level. The model accounts for social activity patterns and weather dependent processes including building heating and renewables, thereby simulating variations in demands, supplies and net network load over seasons, days and years. This simulation thereby informs the electricity industry about investments and operations planning. Although the first focus is electricity, the model calculates heat and gas demand and can simulate district heat load.