Regarding energy use, it seems to me that solar thermal is the way to go; you can keep your tank pretty hot during the day and if it's properly insulated, should minimize energy use at night. If you're showering early in the morning, though, you'll probably be burning some fossil fuel, but if you can time your showers to be mid-day or evening, would be optimal.
In Phoenix, probably every fifth house has solar hot water; it just makes economic sense. When we traveled in the Middle East (mainly Israel), every house and apartment building seemed to have solar hot water. If it makes economic sense (and the tax credit also helps), people will adopt it. I'd be reluctant to support some kind of mandated regulatory approach that forces a less pleasant shower experience on everyone; as others here have mentioned, the shower is one of life's great luxuries.