We recommend heat pump water heaters over solar water heaters due to cost, efficiency, and reliability.
In the more distant past, 10 or more years ago, it was considered more cost-effective to use solar thermal heating for water heating.
Recently, due to the rapid fall in cost of solar electricity systems, and the rise in efficiency of heat pumps, water heating via solar photovoltaic panels that are paired with electric heat pump water heaters has become more cost-effective.
This shift from "heat water directly by the sun" to "use the sun to make electricity, then use the electricity to heat water" was also informed by what has been relatively poor reliability of solar thermal water heating systems - a situation some of our clients have unfortunately encountered.
The one situation QuitCarbon knows of that is still optimal for solar thermal water heating is in larger multifamily and commercial settings, where there is a need for LOTS of hot water, AND there is on-site maintenance staff that can keep the solar thermal systems maintained. Even those sites will likely shift to "make electricity with PV panels, then use heat pumps" in the coming years as the “all electric” approach continues to fall in cost.