It is understood that electricity generation from the waves and tides can be temporally and spatially offset from other, more established variable renewables, such as wind and solar. However, it is less well understood how this offsetting can impact on power system operation. A novel modelling framework has been developed to quantify the potential benefit of including higher proportions of ocean energy within large-scale electricity systems. Economic dispatch modelling is utilised to model hourly supply–demand matching for a range of sensitivity runs, adjusting the proportion of ocean energy within the generation mix. The framework is applied to a 2030 case study of the power system of Great Britain, testing installed wave or tidal stream capacities ranging from 100 MW to 10 GW. For all sensitivity runs it has been found that ocean energy increases renewable dispatch, reduces dispatch costs, reduces generation required from fossil fuels, reduces system carbon emissions, reduces price volatility, and captures higher market prices. For example, including 1 GW of wave displaces up to £137M and 128 ktCO2 over the year of dispatch modelled. Similarly, 1 GW of tidal stream displaces up to £95M and 87 ktCO2. When including 10 GW of ocean energy, dispatch costs reduce by up to 7% and carbon emissions reduce by up to 29%. This analysis has included the development and publication of open source models of the Great British power system. © 2023 The Authors