Judging from the picture: single-family home with no access to public transport. I’m guessing my emissions from a small apartment close to urban rail is still lower than theirs, even if I heat with gas.
I’m not saying it’s a useless project, but the root problem in America is urban sprawl, making almost every aspect of life overly wasteful, in material and energy.
Given how close heating and residential gas use emissions are to personal vehicle emissions it’s likely to come down to efficiency of the cars involved and if their solar powered or not.
I wonder if they also have awnings over south facing windows. It's an old practice (in the Northern Hemisphere) to reduces summer sunlight but allow the winter sun to come through the windows
The Passive House concept by itself can scale up. However, it is expected that you might need to bring in more power from elsewhere at one point when solar alone isn't sufficient to cover local demand.