Floating solar cools lakes and reservoirs, but coverage decides who it helps
Floating solar panels cool the lake or reservoir they float on, but whether that cooling helps or hurts the fish living there depends on how much of the water surface the panels cover. A new study modeled, rather than directly measured, conditions in 11 reservoirs across 6 states, Oregon, Ohio, Washington, Idaho, Tennessee and Arkansas, over 2 month periods in both summer and winter. Evan Bredeweg, then a researcher at Oregon State University, led the work with the United States Geological Survey, a federal science agency. The panels cooled the surface water in every reservoir the team modeled. What that cooling does to the fish underneath is where the answer splits.
Different reservoirs are going to respond differently based on factors like depth, circulation dynamics and the fish species that are important for management.
Evan Bredeweg, lead researcher, then a postdoctoral scholar at Oregon State University. Source 1.
Cooler water below the panels also raises panel efficiency, the share of sunlight a solar panel actually turns into electricity. The study found that gain runs from 5% to 15%, depending on the reservoir.
Oregon State University and the United States Geological Survey state this as a range from cooler water underneath the panels, not 2 disagreeing figures, so both ends are drawn from the same source.
Show the numbers
| Panels over water | 5 to 15% |
The same cooling produces a clear benefit for cold water fish species, salmon among them, only once the panels cover more than 50% of the reservoir surface. Below that share, or in a reservoir managed for warm water species instead, the identical cooling can work against the fish it was modeled to help.
There's no one-size-fits-all formula for designing these systems. It's ecology, it's messy.
Evan Bredeweg, lead researcher, Oregon State University. Source 2.
The effect matches what smaller studies already found
The pattern is not unique to this one study. A 2024 review of floating solar research covering many smaller, lower coverage installations found the same localized cooling below panels, but judged the effect on plant and animal life minor at that scale. Full surface coverage changes more than temperature. A separate 2021 review compared floating panels to the closest comparison available, ice covering the water in winter, and found published research puts the light blocked from reaching water under full ice cover at 53% to 82%.
Ice cover is a comparison the 2021 review used to gauge how a full surface cover changes conditions in the water below it, not a measurement of floating solar panels themselves.
Show the numbers
| Ice covered water | 53 to 82% |
That same 2021 review also traced a single 40% coverage floating array on a Taiwan irrigation pond that measured 0.77 degrees Celsius of winter cooling and 1.4 degrees Celsius of summer cooling, a real reading from 1 site rather than a model. The review also surveyed floating solar operators directly and found only 15% had ever tested the water quality of the reservoir their panels cover. Those operators reported no observed ecosystem impacts anyway, a gap between how little gets measured and how confident the industry sounds about it.
Site specific testing, not a blanket answer
That's the reason that we try to reinforce the need for site specific impact evaluations once we consider putting these floating panels on a large scale.
Ivan Arismendi, co author, Oregon State University Department of Fisheries, Wildlife and Conservation Sciences. Source 3.
None of the numbers above come from counting live fish in the water. Every figure in the 2025 study describes a model of temperature and surface conditions, not a field survey of a fish population before and after panels went in, a limit the study authors state themselves.