A cloudy winter day, but I could see the contour of the sun behind the clouds. I measured around 12000 lux peak. For this test a geared up and tested two of my "new" 7.5W panlels. You can read more about them at http://cottonpickersplace.com/ . I think these panels are very good and they come in many sizes. I prefer panels that are waterproof and has exposed soldering points. I'm evaluating if the slightly smaller panels (1.6x2dm) or this size (2.8x2 dm) is the best.