How efficient are solar panels?

Solar panels are rated for their efficiency in a lab setting. In the real world, there are numerous factors that affect a panel’s efficiency.

the array’s orientation
your roof’s pitch
any obstruction of even a portion of the panel by shade
panels need to be hosed off on occasion as a panel’s efficiency can be slightly ower if it accumulates too much solid dirt such as bird droppings, mud
panels typically require a bit of air flow underneath them to avoid extreme heating conditions