A solar panel is comprised of solar cells that convert light energy, called photons, into electricity. According to the book "Power from the Sun," solar lights function best and shine brightest in full sunlight, as the panel receives higher levels of photons.
In light cloud cover or shade, a solar panel still receives filtered sunlight and photons, allowing it to produce energy and power the light. According the book, "Solar Power Your Home for Dummies," solar panels in light to medium cloud cover receive approximately half the energy of a panel in full sunlight.
According to the book, "Solar Power," in heavy cloud cover or dark shade, a solar panel functions at approximately 20 percent of its normal capacity. So long as the sky is not completely dark, solar lights will continue to function.
Some solar lights come equipped with rechargeable batteries that retain absorbed power to keep the light running at peak capacity in all conditions.