Hello my lovely aunt 
Thanks for posting, great questions! I will get to them all, but first I think we’re missing the most important part of the system, the inverter. Have you got details of what inverters they are proposing? Given your roof shading, I am hoping they are proposing to use Enphase micro inverters. We have details on Enphase here, and why micros are better in your situation:
Also given your shading issues, I would recommend the LG Neon. We have an article on the difference between the models (see below). @Svarky could you clarify the difference between the Neon 2 and the Neon R? Looks to me like the Neon R has a higher efficiency, but Neon 2 may offer better performance in shade? LG Neon and Sunpower Maxeon are really head and shoulders above others in terms of performance in shaded areas. That may be a slightly annoying answer given they are certainly the most expensive option, but I think worth it.
We don’t get much Silfab panels in Australia, so don’t have feedback on that, but LG Neon will be a better bet than Silfab or QCells with shade.
It’s surprising that the installers had such different measurements of pitch and azimuth. It might help in deciding which installer is more competent, by finding out what your actual pitch and azimuth is and seeing who was closer.
Wow, they really are very big differences in system size that they proposed. Please see the chapter in our Guide to Buying Solar on How big a system should I get, that should help you work out which size is more appropriate. Let us know if you have further questions. It would be useful to know how many kWh you use per day on average (should be on your energy bill).
Regarding the tree shading, it’s hard to give feedback on that without seeing the site, or aerial images throughout the year. From memory, there’s a lot of beautiful trees, don’t cut them down! If anything, reduce the system size so that you can install panels just in the area on the roof that has minimal shading. Given you have deciduous trees, I would have thought parts of the roof that have good sun throughout the day in the summer months should be fine to put panels on. They may get partial shading during autumn/winter, but the micro inverters and Neon panels will help to still get decent generation.
Panel weight and wattage don’t matter in short. It’s really about the overall size of the system. However, if you do find that you have limited roof space to use, efficiency may come into it, because you can get more panels in less space. It’s really not going to make much of a difference though, especially if you are choosing between Neon R and Neon 2. A 2% difference won’t make a big difference in what you decide. If you want more info on that, we have an article on panel efficiency here:
Good luck, and please keep us posted with your progress.
Marty
Solar Installers Northern Virginia
Some other installers that have good reviews on the web are:
- Nova Solar, Inc. | Falls Church, VA
- Ipsun Solar | Fairfax, VA
- SolarNova | Hamilton, VA
- Continuum Energy Solutions | Alexandria, VA
- Prospect Solar LLC | Sterling, VA