I wondered this a long time ago too (if solar panels cause more heating by absorbing more), but actually they still reflect a fair portion of the suns energy. [This article](https://www.treehugger.com/ask-pablo-do-solar-panels-contrib...) says earths average reflection coefficient is ~0.35, and that cities are more in the range of ~0.1. Solar panels are ~0.3, so much better than many existing roof materials and of cities in general.