Does Floating Solar Reduce Evaporation? The Evidence

Farmers who fill an irrigation reservoir over the winter watch a good share of it disappear in a hot summer, some to the crop and some to the sky. Floating solar is often said to cut that loss. Here is what the research actually supports, what it does not, and how to weigh it for a farm reservoir.

What the review found

The most useful single source is a 2024 review by Sofia Rocha, Alona Armstrong, Stephen Thackeray, Rebecca Hernandez and Andrew Folkard (“Environmental impacts of floating solar panels on freshwater systems and their techno-ecological synergies”, Environmental Research: Infrastructure and Sustainability). The team, from Lancaster University and the UK Centre for Ecology & Hydrology, gathered 422 pieces of evidence on the physical, chemical and biological effects of floating arrays.

The strongest body of evidence — 114 pieces — supported what the authors call the water-use-efficiency synergy: panel shading lowering evaporation, or saving water. The review also lists the potential to reduce the frequency of algal blooms among the reasons floating solar is attractive.

Why shading cuts evaporation

Evaporation from open water is driven largely by sunlight warming the surface and by wind carrying moist air away. Panels on floats intercept sunlight before it reaches the water and shelter the surface beneath them, so the covered part of the reservoir loses less. The saving scales with how much of the surface is covered — which is also where the caution comes in.

What the evidence does not settle

The same review is blunt that “understanding of the ecological consequences for water bodies is very limited”, despite a wide range of potential impacts. Covering water changes the light reaching it, its temperature and how it mixes, and those changes ripple into oxygen levels, algae and everything that lives in it. For a drinking-water reservoir or a wildlife lake those are serious questions. For a lined farm irrigation reservoir the stakes are usually lower, but the answer is the same: cover part of the surface, not all of it, and ask for an ecological survey and monitoring as part of the planning consent.

What it means for a farm reservoir

Three practical points follow:

1. Water kept is water you do not have to pump in again. Many irrigation reservoirs are filled under an abstraction licence, often in winter. Less summer evaporation stretches that licensed volume further — our guide to water abstraction licences on farms explains the thresholds.
2. The generation fits the season. PVGIS puts a south-facing array at a low 10° tilt near Norwich at about 926 kWh a year per kW, with 53% of it from May to August — when the irrigation pumps run.
3. It is not a substitute for the engineering. Moorings that cope with draw-down, protection for the liner, and — on larger reservoirs — sign-off from the reservoir’s panel engineer all matter more to a farm than the evaporation saving.

Thames Water’s 6.3 MW array on the Queen Elizabeth II reservoir, with just over 23,000 panels, was built to help power a water treatment works — evidence that the technology works at scale in UK conditions, even if a farm reservoir is a fraction of the size.

For the full picture on costs, consents and design, see our guide to floating solar on farm reservoirs.

Sources

• Rocha, Armstrong, Thackeray, Hernandez and Folkard (2024), “Environmental impacts of floating solar panels on freshwater systems and their techno-ecological synergies”, Environmental Research: Infrastructure and Sustainability 4, 042002.
• European Commission JRC, PVGIS v5.3 (queried 6 October 2026), Norwich, 1 kWp, south-facing, 10°, 14% losses.
• WaterBriefing, “Thames Water installs Europe’s biggest floating solar panel project” (15 February 2016).