The tank water temperature does not increase more than two degrees during the day. If you notice it's not so much the water that is getting the sunlight, as just her basking area.
Besides. I'd rather her get REAL sunshine and fresh air when she can. That artificial stuff is just for when it's raining.... I'm a naturalist

One day I hope to maker her an outdoor pond. I will be getting her a kiddy pool this summer for the backyard.
And tank cleaning is SO easy. I just syphon out the water out the window and walk away until it's done. Then I sling the hose from the garden through the window. So easy. And I let her trample around the garden while her tank fills up.