Extrapolation. Use the time on the satellite photo to get the time reference, and thus where the sun is and how much sunlight going at a direction. Use the location and direction of the house get a sense of the facing of the roof. Use color level of the bright and shade sides to estimate the roof pitch incline. Given the roof's area, direction, and pitch, it's not hard to estimate the sun light received.

Far easier, they have 3D models of all the buildings already, as well as weather info. They just need to determine what is roof, and calculate the exposure.