There are a few other things to consider with underground houses...
While the structure itself may offer increased protection against severe weather, basements and underground structures must be designed to resist flooding and to be easily drained. There also must be enough structure above land to allow you to enter/exit even if the surrounding land is covered in a few feet of water. And, of course, the above-ground entrances are susceptible to tornado and hurricane damage. I'm not sure how many tornadoes and hurricanes hit NH, but probably not many compared to other areas of the Union, so don't fret too much.
I keep hearing about how underground homes have the temperature regulated by the heat stored in the earth, to the degree that it takes "virtually no energy" to heat/cool the home. Well, I've lived in Minnesota for over 25 years and I will attest that I've never seen a basement that didn't require quite a bit of external heat after 3-4 months (or 3-4 days) of sub-zero temperatures, and that's with a 70-degree heated house sitting on top. If I remember right, the earth (below the frost point which can be many feet) maintains an average temp around 50 degrees F. This means in the summer when it is 90 outside, the home will be cooled slightly (but not to 50 degrees) and when it's 10 degrees it will be warmed slightly (but not to and certainly not more than 50 degrees). Expect to use quite a bit of energy to heat your home in New Hampshire winters no matter how deep you dig.
Another concern with any basement or underground structure is radon emissions from the ground. I'm no eco-wacko, but I understand radon can be very unhealthy if you're continually exposed to it. Make sure the home is built with radon shielding in mind.
As for solar-powered vehicles, as far as I know there are no commercially-available solar cars, and all the experiments have shown that it's pretty much impossible to build a car that's durable enough for daily use yet light enough to be propelled by the meager energy from the solar panels which would fit on the vehicle. Your best bet at this point would be a hydrogen hybrid, and it will be quite expensive to run, as you will be among the pioneers of fuel cell technology. Or consider an electric-powered vehicle which you could charge with a larger array of solar panels at home. This, too, will not be cheap. Cars use nearly as much energy as an entire home (sometimes more).