Since the consensus here is that UV units are useless, when installed on outdoor pools . . . we really haven't put any effort into learning more.

Outdoor pools in AZ get FAR more UV from sunlight than even the largest residential units are likely to provide. If it *was* 'working' and we were trying to trouble shoot problems with your, we'd have asked you to disconnect it.

Sorry you got snookered. But, don't conclude your builder is 'ripping you off' -- most likely he's been snookered, too. (Of course, he's not paying for it!)