Essentially all of it turns into heat except the part that is (immediately) reflected back into space -- and solar panels are darker than most of the (non-grassy or only sparsely grassy, and the grass is golden, not dark green or on the roof of a building) sites where solar panels are typically sited, which means they reflect less back into space. Ergo, erecting a solar panel adds heat (but again it is negligible compared to the reduction in co2 emissions).