[KennyBlanken]

Solar hot water hasn't been viable for a decade or more; the ROI is piss poor. Efficiency falls as the water heats, and once your storage container is hot enough, the panels are useless.

At least in residential and commercial installations, you get a much higher ROI by putting in solar electric, using the electricity to power your home/facility, and dumping the excess into the grid to earn money/credit.

Resistive heat storage is also a thing these days; hundred-plus gallon tanks that will take power from the panels if it's more cost effective than returning it to the grid or the grid doesn't have the capacity to take it. That water then feeds a second water heater which brings it up to the final temperature, if necessary.

The efficiency relative to area doesn't really matter, as rooftop space is rarely at a premium.