Rapid expansion of wind and solar capacity in Great Britain presents challenges for managing electricity systems. One concern is the reduction in system inertia during periods where renewables provide a high proportion of demand which has led to some networks imposing system nonsynchronous penetration limits. However, given the lack of operational data, the relationship between\ud renewable generation and demand for the full range of meteorological conditions experienced in Great\ud Britain is poorly understood. This study uses reanalysis datasets to determine the proportion of\ud demand from renewable generation on an hourly resolution for a 36-year period.\ud The days with highest penetration of renewables tend to be sunny, windy weekend days between May\ud and September, when there is a significant contribution of both wind and solar generation and demand\ud is suppressed due to human behaviour. Based on the current distribution of wind and solar capacity,\ud there is very little curtailment for all system non-synchronous penetration limits considered. However,\ud as installed capacity of renewables grows the volume of generation curtailed also increases with a\ud disproportionate volume occurring at weekends. The total volume of curtailment is highly dependent\ud on ratio of wind and solar capacity, with the current blend close to the optimum level.