Abstract. The good confinement, high beta, high bootstrap currents and low halo currents achieved in spherical tokamaks (STs) make them potentially attractive commercial power plants. The plasma and engineering parameters of a conceptual ST power plant have been iterated to give a consistent design based on a highly elongated, double null configuration. Steady-state operation is feasible with 50 MW of neutral beam injection or by using RF schemes e.g. Electron Bernstein Wave. Exhaust power densities are relatively high but can be handled using a continuously renewable target comprising a cascade of 2-3 mm diameter SiC pebbles which is being developed. A helium cooled, ceramic pebble bed blanket with beryllium multiplier generates the required tritium and achieves 43% overall thermal efficiency. The layout of the torus hall is constrained by the need for maintenance access from below and for the TF coil power supplies to be near the torus to reduce power losses. Routine maintenance is achieved by lowering the simple, robust centre column into a hot cell beneath the load assembly for replacement. This work shows the ST to be a strong candidate for future economic power generation by fusion.