Nearly 5,000 solar panels at 2,500 meters altitude, atop a dam and without road access: Muttsee's solar installation in the Swiss Alps generates 43% of its electricity in winter, when the country needs it most

Solar panels have long been part of our daily lives, integrated into our landscape. If we stop to think about it now, we would likely place them on urban rooftops or on vast plains in the countryside. Certainly, no one would imagine placing them on a dam high in the Alps, at an altitude of 2,500 meters, with no access road, snow for six months of the year, and temperatures cold enough to freeze the water in the pipes. Yet, precisely there, 4,872 solar panels have been operating since 2022. And they are not there by chance. They are there because, in winter, they generate nearly half of their annual electricity output. Exactly when Switzerland needs it most.

The plant is called Muttsee, located in the canton of Glarus, and occupies the wall of the dam that bears its name. It was built by Axpo, the Swiss utility, and the panels are not installed on the ground: they are mounted on the south-facing concrete wall, with a tilt of 51 to 65 degrees to allow snow to slide off and prevent accumulation. There is no road access. All materials, from the panels to the aluminum structures, were delivered by helicopter. It was a slow, expensive, and complex construction project. And yet, the performance figures from its first years of operation have surprised both insiders and outsiders.

According to a study by the École Polytechnique Fédérale de Lausanne, which measured production between October 2022 and September 2023, the plant generated 1,434 kilowatt-hours per installed kilowatt-peak over the course of the year. Of this total, 616 kilowatt-hours were produced during the winter semester, from October to March. In other words, 43% of the entire annual output was concentrated in the cold months. To put this in context: at a low-altitude facility, such as the reference site in Payerne, the winter production per unit of capacity was nearly four times lower. In winter, Muttsee produces approximately 3.8 times more per installed kilowatt than a conventional plant in the valley.

Why it works in winter

The explanation has three pillars. The first is altitude. Muttsee sits at 2,500 meters, above the layer of fog and low clouds that blankets the Swiss valleys for much of the winter. While Zurich or Bern may experience gray, sunless days, the sky above the dam is clear. The south-facing orientation of the wall does the rest: the panels receive direct radiation for more hours than any installation in the valley floor.

The second is the cold. Solar panels do not perform better in heat. On the contrary: silicon cells lose efficiency as temperatures rise. On a summer day, a panel in the plains can reach 60 degrees and lose 15 to 20 percent of its output. At Muttsee in winter, the panel is at zero degrees or below. The cell operates within its optimal range. The cold, which seems like an enemy, is in fact an ally.

The third factor is snow. The panels are bifacial, meaning they can capture light from both sides. Snow on the ground reflects solar radiation toward the rear side, which adds to power generation. At Muttsee, the dam’s geometry limits this effect, but on sunny days with snow-covered ground, the contribution is present. Furthermore, the steep tilt of the panels causes snow to slide off, preventing it from covering the active surface—a major challenge for solar installations in mountainous regions.

The problem Muttsee solves unintentionally

Switzerland faces an energy challenge that goes beyond its reputation as a green country. In winter, it consumes significantly more electricity than it produces. Hydroelectric dams, which have ample capacity in summer, see their water flow decrease because the water is retained as snow in the mountains. Solar panels in cities, facing short days and fog, generate little power. Consequently, the country ends up importing electricity from France and Germany, much of which is generated using fossil fuels. This winter gap is set to widen in the coming years as Switzerland shuts down its last nuclear power plants and electrifies heating and transportation.

Muttsee does not solve this problem. It generates 3 gigawatt-hours per year, a modest amount for a country that consumes approximately 60,000 gigawatt-hours. However, it demonstrates something important: that mountain solar can produce power when it is needed most. It is not an engineering curiosity. It is proof that there is a niche, a space where solar energy and altitude complement each other. And that niche could grow.

What the headlines don't tell you

Here, honesty is called for. Muttsee is an exceptional case for several reasons. The first is that the dam already existed. The grid connection was already in place. The concrete wall was already oriented to the south. The panels leveraged infrastructure built decades earlier for a different purpose. This is what makes the project viable. Building a solar plant from scratch at 2,500 meters, using helicopters and without a road, would cost significantly more and would likely not be economically feasible.

The second is that the data covers only a single year. Moreover, it was a year with specific conditions. The study acknowledges this: the figures cannot be extrapolated as a long-term trend. There may be winters with more fog, less sunshine, or heavier snowfall. It will be necessary to observe how the plant performs over the course of a decade.

The third challenge is maintenance. The panels are installed on a dam wall. Any repair, cleaning, or inverter replacement requires climbing to that location. In summer, access is possible on foot or by off-road vehicle. In winter, it requires a helicopter or mountaineering skis. This increases operational costs and complicates the work for technicians. Axpo has not disclosed the cost of maintaining the plant, but it is unlikely to be cheap.

The fourth issue is the visual impact. The Muttsee dam is a concrete monument situated in a protected alpine landscape. The panels partially cover it. Some consider it an elegant integration. Others view it as an intrusion. The debate exists and is not trivial.

An idea that can be replicated

What makes Muttsee interesting is not that it is unique. It is that it can be replicated. Switzerland has hundreds of mountain dams, many of which face south, and many are already connected to the grid. Not all are suitable. But some are. And in a country that desperately needs to generate electricity in winter, every dam equipped with panels is a small victory.

Axpo is already studying other sites. Other Swiss utilities are doing the same. And in Austria, Italy, and France, similar projects are underway. Mountain solar will not replace valley solar or hydropower. But it can be another piece of the puzzle. A piece that produces exactly when the others fail. And in a power system that is increasingly dependent on weather conditions, that is precisely what is needed.