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Photovoltaic greenhouses have been claimed to be a solution to cover the energy demand of the protected crops sector. Thus, there is a need to know what is the maximum percentage of shading produced by roof-top photovoltaic panels that does not affect crop yields. The present study analyzes the effects of increasing percentages of shading in a greenhouse tomato crop located in the southeast of Spain. For this study, photovoltaic panels have been simulated with opaque sheets located in the roof-top of a north–south oriented greenhouse. Three treatments of top roof shading percentage (15%, 30% and 50%) where studied and compared with the control treatment without shading (0%). During the study, parameters registered were radiation, temperature, pH and electric conductivity of the substrate, crop yields and fruit quality. Results of the analysis show that higher percentages of shading in the roof-top of greenhouses reduce so much available radiation for the crop causing a reduction in the yield and fruit quality, even in Mediterranean areas where radiation is not a limiting factor. [ABSTRACT FROM AUTHOR]