Well, Jimh, now the joke’s on me, since I had you post here. I thought you were trying to compare between electric and propane, which I’m probably not qualified to do.
But on this subject, I can tell you that heat-pump water heaters are about twice as efficient as a regular electric. However, there can be issues. Happily, you can see some of them pretty easily.
The newest section of my website is The Future of Water Heating. If you go there and check out Chris Gray’s presentation, you’ll find out some of the things Southern Power ran into when it tried out these heaters in various situations.