Solar farms need a lot of flat, open space, and good land is getting harder to find. So a growing number of solar plants are skipping the land altogether. They sit on plastic floats on reservoirs, quarry lakes, and irrigation ponds, tied down with anchors and cables. Floating solar started with a handful of small test rigs less than 20 years ago. Today the biggest plants hold hundreds of thousands of panels. Here is how it works, what the water adds, and what can go wrong.
A solar farm on a raft
A floating solar plant uses the same panels you would see on a roof or in a field. The difference is what holds them up. Instead of metal posts driven into the ground, the panels are bolted onto rows of hollow plastic floats, a bit like a very large dock. The floats are clipped together into big sections, sometimes called islands.
Those islands cannot just drift around, so they are held in place by mooring lines. The lines run to heavy anchors on the bottom of the lake or to fixings on the shore. Cables carry the electricity back to land, where it is converted and fed into the grid or into a nearby factory or water plant.
Builders usually put the whole thing together at the water’s edge. Workers bolt panels onto floats on a ramp or a beach, push the finished rows into the water, and then tow them out by boat to be hooked onto the mooring lines. Compared with a land-based solar farm, there is very little digging and no concrete foundations, so the whole plant can, in principle, be taken apart again.
From a winery pond to a reservoir in Java
The first floating solar test system went in at Aichi, Japan, in 2007, built by the country’s National Institute of Advanced Industrial Science and Technology. A year later, in May 2008, Far Niente Winery in Napa Valley covered part of its irrigation pond with 994 panels on 130 pontoons, about 175 kilowatts in all. The first plant of a megawatt or more opened in Okegawa, Japan, in 2013.
For a few years, these plants stayed small. Then the cost of solar panels kept falling, and the idea took off, mostly in Asia. Worldwide, installed floating solar grew from about 1 gigawatt in 2018 to about 13 gigawatts by 2022. In 2022, the Chinese utility Huaneng opened a 320-megawatt floating plant in Dezhou, in Shandong province.
One of the best-known big plants floats on the Cirata Reservoir in West Java, Indonesia, between Jakarta and Bandung. The reservoir was built in 1988 for a hydropower dam that can make about 1,000 megawatts. In November 2023, Indonesia’s president opened a solar plant on top of it with more than 340,000 panels, arranged in 13 floating islands. It is rated at 192 megawatts at peak and was built by the state utility PLN and Masdar, a renewable energy company from the United Arab Emirates. In its first year, it sent more than 267 gigawatt-hours of electricity into the grid, and it still covers only about 4 percent of the reservoir.
The United States has been slower. The largest floating array in North America when it opened, in January 2023, sits on the Canoe Brook reservoir in Short Hills, New Jersey. Its 16,510 panels cover 17 acres of water and supply about 95 percent of the power used by the water treatment plant next to it.
Why put panels on water?
The first reason is simply space. A reservoir or a flooded gravel pit already exists, and covering part of it does not take farmland, forest, or open country. In crowded places with high land prices, like Japan, Singapore, and parts of Europe, that alone can make floating solar worth the extra trouble. On farms, there is another way to share space, which is to grow crops in the partial shade of raised panels, and we looked at that in our piece on growing food under solar panels.
The second reason is temperature. Solar panels work less well as they heat up. A panel baking over a hot field or a dark roof loses a little of its output, while one sitting just above water stays cooler. The gain is real but modest. A 2024 global study in the journal Nature Water assumed floating panels run up to 10 percent more efficiently than panels on land, and field comparisons often find smaller gains of a few percent.
The third reason is the water itself. Panels shade the surface and block some of the wind, and both of those slow evaporation. That matters a lot in dry places, where reservoirs can lose huge amounts of water to the air. A 2023 study in Nature Sustainability estimated that covering 30 percent of the surface of more than 114,000 reservoirs around the world could save about 106 cubic kilometers of water a year. The shade can also cut down on algae, which need sunlight to grow.
The last reason is the power lines. Many reservoirs already have a hydropower dam with a grid connection. Putting solar panels on the same water lets both plants share those lines, and the dam can hold back water on sunny days and release it at night. In Cirata, the plant’s operator has pointed out that solar helps most in the dry season, when the hydropower plant has less water to work with.
How a lake plant is built
Germany’s largest floating plant when it opened in May 2022 gives a sense of the details. It floats on the Silbersee III, a former quarry lake in Haltern am See, next to a quartz sand works run by a company called Quarzwerke. The plant’s 5,800 panels were bolted onto 360 floating sections on the shore and taken out by boat. It is rated at about 3 megawatts and is expected to make about 3 million kilowatt-hours a year. Around three quarters of that is used by the factory itself.
Even this mid-sized plant needed serious anchoring. It is held in place by 24 special anchors set between 13.5 and 23 meters deep, and a breakwater almost 300 meters long protects it from waves. The panels face east and west at a low tilt of 12 degrees, which spreads the power out across the day and keeps them low to the wind. They are glass on both sides so that some light still reaches the water below. Altogether, the plant covers about 1.8 hectares, around 2.3 percent of the lake.
When the wind wins
Water is not as calm as it looks, and wind is the biggest risk to a floating plant. Japan learned that the hard way. In 2018, Kyocera opened a 13.7-megawatt floating plant with about 50,000 panels on the Yamakura Dam reservoir in Chiba Prefecture, near Tokyo. It was held by 420 anchors and 823 mooring lines.
On September 9, 2019, Typhoon Faxai hit Chiba with gusts up to 207 kilometers an hour (about 129 miles an hour). Seven anchors on the south side of the array came loose. With those gone, the strain shifted onto the plastic bolts that held the floats together, and they began to break one after another. The single big island tore into three pieces and was pushed against the shore. Panels piled up on top of each other, short-circuited, and caught fire. Roughly two thirds of the plant was destroyed.
A government investigation found that the main cause was anchor failure, and that the shape of the array made things worse. When the plant was rebuilt, it was split into six smaller islands so that the forces on any one part would be lower. It went back into service in 2021, about two years after the storm.
The industry has learned from cases like that one. In 2024, some floating plants rode out Typhoon Yagi in southern China and Hurricane Milton in Florida without damage, according to their operators. But the lesson stands. On water, the anchors and connections matter as much as the panels.
The catches
Floating solar usually costs more to build than a plain solar farm on land, mostly because of the floats and the anchoring. A researcher at the U.S. National Renewable Energy Laboratory put the extra cost at roughly 10 to 25 percent in 2023. Repairs are harder, too, since workers have to reach the panels by boat or by narrow walkways, and every part has to resist damp and corrosion for decades.
The bigger open question is what large plants do to the life in a lake. Shading the water can reduce algae, but it also cuts the light that water plants and tiny drifting organisms need. Panels block the wind, which stirs the water and mixes oxygen into it. The authors of the 2024 Nature Water study wrote that scientists still cannot predict whether a big floating plant will make a lake warmer or cooler overall, or how that will ripple through the fish and the food web. The authors suggested starting with artificial water bodies, like quarry lakes and reservoirs, rather than natural lakes.
People also have feelings about their lakes. In the Netherlands, some residents near one lake told researchers they worried floating panels would spoil the view and get in the way of recreation. Rules can limit how much of a reservoir can be covered. Indonesia, for example, raised its cap to 20 percent of a reservoir’s surface in 2023, which opened the door to expanding Cirata.
How much could it add up to?
Potentially, quite a lot. The Nature Water study looked at more than a million lakes and reservoirs around the world. After ruling out those in protected areas, those far from towns, those that freeze for much of the year, and those that sometimes dry up, it was left with 67,893 water bodies. Covering 10 percent of each of them could make about 1,302 terawatt-hours of electricity a year, In a few countries, such as Ethiopia, that would be more electricity than the whole country used in 2021.
In the United States, the National Renewable Energy Laboratory looked only at reservoirs controlled by federal agencies or licensed for federal hydropower. It estimated those alone could host between 861 and 1,042 gigawatts of floating solar. That is a technical ceiling, not a plan, since cost, permits, recreation, and wildlife would rule out much of it. Still, it shows that the space is there.
The bottom line
Floating solar is no longer a curiosity. It works best on water that people have already reshaped, like hydropower reservoirs, drinking-water reservoirs, and old quarry lakes, especially where land is scarce or the sun is strong. It costs a bit more, it needs careful anchoring, and its long-term effects on lakes still need more study. But on a hot, dry day, a raft of panels that keeps itself cool and keeps the water from evaporating is hard to argue with.
The next time you drive past a reservoir, look at the surface. More and more of them are starting to shine.
Further reading and a tiny floating solar plant of your own
Here Comes the Sun: A Last Chance for the Climate and a Fresh Chance for Civilization — Bill McKibben’s short, readable case that cheap solar power has changed what is possible, and why the hard part now is building it fast enough.
Mademax 1.4W Floating Solar Fountain Pump for Bird Baths and Ponds — A small solar panel on a float that drives a little water pump whenever the sun hits it, with eight spray nozzles and four brackets to keep it centered. It has no battery, so it only runs in direct sunlight.