Floating photovoltaics (FPV) refers to photovoltaic power plants anchored on water bodies with modules mounted on floats. FPV represents a relatively new technology in Europe and is currently showing a rapid growth in deployment. However, effects on thermal characteristics of lakes are largely unknown, yet these are crucial for licensing and approval of such plants. Here, we quantify FPV impacts on lake water temperature, energy budget and thermal stratification of a lake through measurements of near-surface lateral wind flow, irradiance, air and water temperatures at one of the largest commercial German facilities, situated on a 70 m deep dredging lake in the Upper Rhine Valley, South-West Germany. Underneath the FPV facility, a 73% reduction in irradiance on the lake surface and an average 23% reduction in near-surface wind speed at module height are detected. A three month data set is then used to set up the General Lake Model and simulate scenarios of different FPV occupancies and changing climatic conditions. We observe that a lake coverage with FPV result in a more unstable and shorter thermal stratification during summer, which could mitigate the effects of climate change. The reduction of water temperatures follows a non-linear relationship with increased FPV occupancy. A sensitivity analysis showed that an increased wind reduction by FPV can have a considerable impact on the thermal properties of the lake. However, measurements only suggest small deviations with regard to the thermal properties of the investigated lake. These findings can be used in approval procedures and allow for a more accurate assessment of environmental impacts of future installations.