Switzerland added a welcome new element to its energy system, in the form of an underground hydroelectric plant capable of storing enough electricity to simultaneously charge 400,000 car batteries, Reuters reports.

The developers of the 2.2 billion CHF ($2.3 billion) Nant de Drance hydroelectric plant in the Canton of Valais, which became operational in July, say the facility works like a giant battery, according to Agerpres.

The plant’s six turbines, located in a cavern 600 meters below the Emosson and Vieux Emosson reservoirs, have a capacity of 900 Megawatts, making it one of the largest pumped storage hydro plants in Europe.

At times when electricity demand reaches its peak, the Nant de Drance power plant produces hydroelectricity. But when electricity production from sources such as wind turbines and photovoltaic panels exceeds demand, the Nant de Drance plant uses the surplus electricity to pump water into the upper Vieux Emosson reservoir.

“In this way, there are some losses, just like any other storage solution, but the efficiency is very good. We have an efficiency of about 80% at a complete cycle”, said the director of the plant Alain Sauthier.

Officially, the Nant de Drance plant, whose construction took 14 years, will be opened next month.

It takes less than five minutes for the plant to switch from pumping mode to electricity production mode. At 360 cubic meters per second, the volume of water passing through the plant’s turbines corresponds to the flow of the Rhone River through Geneva in the summer.

“The amount of energy that can be stored in this facility is simply gigantic, about 20 Gigawatt-hours. This means that thanks to a full tank at Vieux Emosson you can simultaneously charge 400,000 car batteries,” said Alain Sauthier.

Normally, the plant pumps water into the reservoir during the afternoon, night or weekend, and then produces electricity in the morning and evening, when prices are higher, Sauthier said.

“I can say that it is important not only for Switzerland but it is important at the European level because it will contribute to the stability of the network in Europe and to the security of supply in Europe,” added Sauthier.

The inauguration of the plant comes at a time when Switzerland faces the prospect of temporary interruptions in the supply of electricity in some regions, if the energy crisis in Europe leads to disruptions in the production of electricity in winter.