The UK’s historic fossil fuel-based energy system could easily adjust to changing demand by ramping production up or down. This system provided electricity, heat and transportation fuels in a largely dependable and predictable way. Its reliability was underpinned by straightforward energy storage: coal piles (for electricity), pipes and chambers full of gas (for heat), and oil/petrochemical tanks (as transport fuels). But as the climate impacts of fossil fuel use have become clear, the energy system has begun shifting towards low-carbon alternatives—posing new challenges for how we store energy. Unlike fossil fuels, renewable energy sources like wind and solar are variable and less controllable. Solar generation fluctuates with daylight, while wind output depends on weather. These sources don’t always align with energy demand, which itself varies daily and seasonally (although demand for energy currently follows relatively consistent patterns). As a result, the system requires new forms of flexibility to avoid a host of technical, economic and social problems (Royal Society, 2023). Part of this flexibility will need to come from energy storage.