The coatings I know of have to be tested at the time of install and then again in 3 years without cleaning. The levels of reflecting the solar rays are then included in the product rating. Both Energy Star (govt funded) and CoolRoofs (industry funded) have searchable lists of products and ratings. The ratings vary widely, so it's worth checking out an individual product to see how it will perform.

Here is the product search form for CoolRoofs:

http://www.coolroofs.org/products/search.php