The concept of storing renewable energy in stones has come one step closer to realization with the construction of the GridScale demonstration plant. The plant will be the largest electricity storage facility in Denmark, with a capacity of 10 MWh. The project is being funded by the Energy Technology Development and Demonstration Program (EUDP) under the Danish Energy Agency. Pea sized stones heated to 600°C in large, insulated steel tanks are at the heart of a new innovation project aiming to make a breakthrough in the storage of intermittent wind and solar electricity. The technology, which stores electrical energy as heat in stones, is called GridScale, and could become a cheap and efficient alternative to storing power from solar and wind in lithium-based batteries. While lithium batteries are only cost-effective for the supply of energy for short periods of up to four hours, a GridScale electricity storage system will cost effectively support electricity supply for longer periods — up to about a week.