How does pumped storage work?

In French it is called a STEP, a "pumped energy transfer station". Picture two bathtubs, one in the attic and one in the cellar, joined by a pipe and a pump that can also run backwards, like a turbine. When the grid has electricity to spare (at night, at weekends, or when wind and sun are producing a lot), the pump lifts water from the cellar to the attic. When everyone switches on their oven at the same time, the tap is opened: the water runs back down, turns the turbine, and power is back on the grid within minutes.

So a pumped-storage plant does not produce energy, it stores it. It is a battery made of water and granite. About a quarter of the energy is lost on the way, but you gain what matters: electricity available at the exact moment it is needed.

Montézic in figures

  • The attic: Lake Montézic, at about 700 metres above sea level, with 30 million cubic metres of usable water, held back by two rockfill dams (Monnès, 820 metres long and 57 high, and L'Étang, 680 metres long).
  • The cellar: the Couesques reservoir on the Truyère, the one you look down on from Valon.
  • The drop: a little over 400 metres between the two.
  • The machines: four reversible units of about 230 MW as turbines and 210 MW as pumps, installed in a cavern cut into the granite more than 400 metres down. About 910 MW in all, the second-largest pumped-storage plant in France.
  • The rhythm: with all four pumps running, the usable part of Couesques would be emptied in about forty hours. So the plant pumps and generates almost every day.
  • The history: built from 1976 to 1982, the four units were refurbished between 2015 and 2020. If the whole grid goes down, Montézic is one of the plants able to help restart it.

Montézic II, the second plant

EDF wants to dig a second underground plant next to the first, with two new pump-turbines of about 233 MW each: 466 MW more, taking the total close to 1,400 MW. The lakes stay the same; the water will keep going back and forth between Montézic and Couesques. The project was held up for a long time by a disagreement between France and Brussels over hydroelectric concessions. After the 2025 compromise, an exploratory tunnel to check the rock starts in summer 2026 and will take a year and a half to two years. The main works will follow for about five years, with commissioning announced for around the mid-2030s.

Like the first plant, the new one will be dug inside the mountain: the work will take place mostly underground, without disturbing walkers or visitors. For the power grid, it will be one more big battery to keep up with the ups and downs of wind and sun.

Coming to work on the project? Our cottages are right opposite the plant, 600 metres as the crow flies: see our Montézic II accommodation page.

The guided tour of the plant

The tours are led by EDF staff who work at the plant. You start in the information area at the entrance: display panels, films and a scale model of the whole scheme, from the two lakes down to the machines, which makes the path of the water easy to follow. Then, with a helmet on, you set off along the access tunnel. It is almost a kilometre long and leads down to the machine hall, 400 metres below the plateau. The cavern is about 142 metres long, 25 wide and 35 high: the four units stand in a row, and the lights in the vault look like a starry sky. Allow about two hours in all.

  • When: Saturdays and Sundays from May to mid-September, with several tours a day (morning, early and late afternoon).
  • Booking: at least 48 hours ahead, with the tourist information office in Saint-Amans-des-Côts (+33 5 65 44 81 61) or on the Tourisme en Aubrac website. Nobody gets in without a booking, even if there are places left.
  • Who: ages 8 and up. Every visitor must show an identity document.
  • What to wear: closed shoes are compulsory, and bring a jacket. Helmets and safety vests are lent on site.
  • Leave in the car: bags. Photos and videos are not allowed in the machine hall.
  • Meeting point: the plant reception on the D621 in Montézic, 20 minutes before the tour. From Valon it is about ten minutes by car.

Even without a tour, a walk around Lake Montézic is worth it, and the Saut du Chien waterfall is just above the plant.

Our tip

Book early in summer, as places are limited. After the tour, have a picnic on the plateau and spend the afternoon at the Saut du Chien. In the evening, back in Valon, look down at the Couesques reservoir: its level rises and falls with the country's electricity use.