Rooftop photovoltaic (RPV) systems are a crucial means to achieve urban carbon peak and carbon neutrality goals. However, their potential climatic and environmental effects require assessment. This study integrates an RPV scheme into the Weather Research and Forecasting (WRF) model to evaluate its impact on urban temperatures during summer in a typical valley city within Northwest China's arid and semi-arid regions. Valley cities in this area are characterized by complex topography that traps heat and pollutants, coupled with limited water resources and high solar radiation, making them particularly vulnerable to intense urban heat island (UHIs) effects. Understanding RPV impacts here is therefore highly relevant for sustainable development. Key findings reveal: (1) The cooling effect of RPV on urban built-up area air temperatures at 2 m exhibits a non-linear relationship with coverage rate, peaking at 50% coverage for both horizontal (max: 0.228 K) and vertical (max: 0.032 K) cooling. This non-linearity is likely attributable to complex interactions between RPV-induced changes in roof albedo, conversion of solar radiation to electricity (reducing sensible heat), and subsequent alterations to the urban surface energy balance. (2) During the afternoon (14:00), 50% RPV coverage reduced regional average UHIs intensity from-0.51K to-0.55K (heatwave days) and from-0.56K to-0.59K (nonheatwave days), with minimum UHIs values also decreasing significantly. (3) At night, urban built-up areas exhibited stronger heat islands during heatwaves. While RPV's overall nocturnal impact on UHIs was generally small, 50% RPV coverage intensified the nighttime UHIs during heatwaves (regional average UHIs increased from 0.954 K to 0.981 K; peak UHIs rose from 14.840K to 15.378 K), potentially due to retained heat or altered nighttime longwave radiation exchange. This study highlights the nuanced climatic impacts of RPV deployment in valley cities, providing valuable insights for future urban planning and photovoltaic integration strategies.