In the USA heating, HVAC, etc.. account for half of the energy used in residential buildings. https://en.wikipedia.org/wiki/Solar_thermal_energy Direct solar water heating (passive) has been used for quite a while and has a maximum efficiency of solar thermal systems are between .46 & .74. https://en.wikipedia.org/wiki/Solar_water_heating

That compares to a solar panel efficiency of .15; and I guess a lot less when applied to water heating due to conversion. And implementing in dense urban areas, passive is not as viable as active. If we were serious we would first implement passive systems. But we don't. I guess there’s something I don’t understand.