For an off-grid tiny home the ...

For an off-grid tiny home the panels take up most of the roof, so, instead of roofing that area it's being used for air heating-cooling by using them like greenhouse panes sealed for weather over a 3/4" gap to heat quickly, it's an active system with a blower to run when conditions allow and a thermal-mass is used to store it in the crawl space. The cost of the roofing pays for the box & much of the duct work. The site is cold and windy, 6,600-deg-days a year and 15kn average windspeed so having this setup can heat the home with few conditions not allowing it, uses a small electric heater as backup that's installed to heat the main room if you want. For existing homes with a good section of roof it's a different game but can then recommend altering the roof pitch to maximum gain as most won't be steep enough in the USA. As a carpenter I've done this on framed roofs, it's not too bad a job, usually two days to reframe it; it's an option to only do part of a roof this way adding gable walls. Doing this improves output and gives the chance to collect the air under the panels. For crawl space thermal-mass, it's a stack, you dig 18" of dirt & layer 2" soil-contact foam dirt goes back, leveled, radiant-vapor barrier, 1/2" of sand to set concrete blocks on their side to use as ducting, manifolds for that into ducting, then a fabric water tank for the main mass, these come in any size and volume so it's simple to engineer the stack for capacity against heat loss for the daily cycle. For where there's no way to add this build a separate one, it could be done as a deck. Looks able to keep the temps comfy without much input, with so much wind it's all electric, with the thermal mass it's a warm floor on a cold winter day.