The work presented in this paper is an integrated time-dependent model, describing the power demands of the major tokamak components (magnets, cryogenics, heating and current drive, etc.), as well as the power generated. The physics are implemented in OpenModelica and make use of a Python API (Application Programming Interface) to collect inputs, run studies and record outputs. The model cannot be validated against real world data, since there is no operational tokamak in the world designed for electrical power generation. Therefore, the correctness of each submodule (i.e., the magnet model, the cryogenics model) has been verified either from first principles or via verification against data from JET (Joint European Torus) where possible. The model has been used extensively as part of the work on the UK’s Spherical Tokamak for Energy Production (STEP) and has informed decisions on the STEP concept. It is going to be publicly available on GitHub.