Upper Krust's ECL guide (v5) states ECL 42 exactly for Solars.
In general, I disagree with the CR = PC HD rule. I agree that there's a difference between CR and ECL, and that it's pretty consistent everywhere -- PC and NPC differences don't really count all that much, even when it comes to abilities like Wish 1/day. I mean, after all, the ability to forge documents, learn facts about any topic, travel long distances, etc, are pretty useless in a single combat, but quite useful to PCs... and PCs get them!
WotC has a pretty stupid double standard between "monsters" and PCs, I think.
In most cases, CR is roughly equal to 2/3s ECL. Dragons are a notable exception. Solars too, obviously. If we take the ECL for Solars (which was calculated according to some pretty strict rules) and apply the 2/3 rule to it, they should actually be CR 28. This makes sense to me as well. I mean, look at them! They're big and scary and loaded with incredible abilities!