As I have mentioned before, I use Google Sketchup for modeling software, mainly to size and locate south facing windows and overhangs. As soon as our house was closed in I started looking back at the shadows on the south facing walls,and it appeared very accurate. Today I measured where the sunlight actually hit the interior floors and then went back to the model to compare. It is accurate to within a fraction of an inch. YMMV but I am totally impressed with the result. :D

I used sketchup to model and visualize/walkthru our floorplan. Worked great although my slower computers had speed issues when the drawing became complex. I played around a bit with the sunlight feature, looked interesting. As with all simulators, garbage in = garbage out so you would need to be sure of your actual orientation vis the simulated orientation...

Google Sketchup is indeed accurate for determining shadowing of doors/windows given the building latitude and south wall orientation relative to true south, and the door/window orientation relative to the roof overhang. Chief Architect and other similar construction drawing development tools are also accurate in this regard.

However, to properly design a passive solar building you should also account for any terrain obstacles, fenestration SHGC and area, local clear sky optical parameters and local climatic sunshine. Proper passive solar building design is ultimately about accurately forecasting both clear sky and climatic irradiance heat gain BTUs and then designing the buffering interior thermal mass and HVAC system accordingly so you don’t overheat the building or under/over-size the HVAC system.

We have passive solar design tools on our website as does Sustainable By Design.