One of the biggest impacts of Net Zero will be the need to find alternatives to the (unabated) use of fossil fuels with their low cost, large capacity and long duration storage options, which still provide nearly all the flexibility and resilience that balance Great Britain’s energy systems. Currently, the heavy lifting in balancing Great Britain’s electricity and heat sectors is done by natural gas, capable of contributing 3-4 TWh towards managing imbalance daily, and over 100 TWh seasonally. This new research examines future imbalance in a ‘pure’ renewable electricity system, exclusively using various ratios of wind and solar generation, scaled to meet average electricity and heat demand. Imbalance calculations are based on actual measured generation and demand data for each day in the period 2015 to 2019 and not derived indirectly from meteorology. The results show: Future orders of magnitude of system imbalance similar to those in the current system. Electrifying heat would transfer imbalance into the electricity system doubling or trebling the scale of daily and cumulative imbalance levels, depending on the technology chosen. The ‘optimum’ mix of wind and solar for minimising imbalance differs according to the timescale examined and whether the electricity sector is considered alone or together with the heat sector.