I’ve recently taken to referring to the patches of sunlit carpet that our cat likes to bask in as “sun puddles”. Just the name is new for me, I’ve known cats like to recharge their solar batteries for years now. After all, sleeping and keeping a bed from floating away take a lot of energy.
But I digress. Is “sun puddles” a new name for these? I suspect I’m just late on the bandwagon. How long ago did you come across this phrase? 
Just now… 
I once saw a reference to puddles of cats, but not puddles of sun.
I won’t laugh at yours if you don’t laugh at this:
As a kid, my dad called an oil-slick rainbow a “summerset” (or possibly “somerset”).
AFAIK, you are the first to use the term “sun puddles”. Take it, run with it, and rejoice!
You might want to read Robin Wood’s Theory of Cat Gravity, just to know what your cats are doing with those Sun Puddles:
http://www.robinwood.com/Catalog/Prints/PrintPages/CatGravPoster.html
http://www.robinwood.com/Catalog/Books/BookPages/CGBBook.html
I hadn’t heard it before either. According to Urban Dictionary, though, it goes back at least a while.
Google ngram doesn’t return any hits, but Googlng the phrase brings up seveal hits, and the images usually seem to include cats!
I like it.
I haven’t heard it, but dogs also like to lay in sun puddles.
I like to use the term shade pool when walking the dog through the park on a hot, sunny day. Come on dog, let’s head for that next shade pool.