Energy storage is key to secure constant renewable energy supply to power systems – even when the sun does not shine, and the wind does not blow. Energy storage provides a solution to achieve flexibility, enhance grid reliability and power quality, and accommodate the scale-up of renewable energy. Storage systems can help enhance reliability. Deployed together with variable renewable energy like wind and solar, it can help displace costly and polluting fossil fuel-generated electricity, while increasing security of supply. Energy storage systems in the developing world will be a game changer. They will accelerate much wider access to electricity, while also enabling much greater use of renewable energy, so helping the world to meet its net zero, decarbonization targets.