"Attached are some photos (before and after) of a solar DHW system I recently finished.  It is on a roof of a 16 unit apartment building.  It is a drain back system. 

Prior to the installation, the building was using $4,600 annually in natural gas solely for domestic hot water.  A new 95% efficient Polaris gas water heater was installed along with this solar DHW system.  Not counting my labor, or the gas heater installation, the project cost $13,645.    Between the new heater and the SDHW system, gas bills will hopefully be cut by 2/3.  I believe a lot of commercial SDHW applications can make more financial sense than residential applications with the increased economies of scale.  I have adopted commercial solar DHW as a hobby, and would be happy to help anyone with questions. "