Power grids encounter varying levels of demand throughout the day. When demand is light, excess energy is produced. The challenge for energy companies is finding ways to store this energy, so it can go back into the grid when demand is high.

But a company in California, US, thinks it may have the answer. It's developed something called an Advanced Rail Energy Storage System (ARES), which stores and transports surplus energy using a "gravity train".

BBC Click finds out more.

See more at Click's website and @BBCClick.