Energy system models are generally written in algebraic modelling languages like AMPL or GAMS. This both limits the extendibility of the models due to the structure of the languages and can be seen as a threshold due to the license costs for the languages. Julia with JuMP is an excellent alternative as it is both open source and allows for flexibility in model structure by using multiple dispatch. The model is a novel energy system model framework with easy extendibility and high flexibility.