I'll play devil's advocate a little.
Appreciated!
You're not taking into account how much you have to pay to put in enough solar that you can keep an electric car charged. Since you can't get paid out for a surplus of solar power, you might be better off having slightly too little capacity and paying for that tiny bit extra you use. Your use should be elastic enough that you can turn some things off if the grid is down and still have solar keep the important stuff working.
The system will be slightly undersized with an electric car - I do expect to still buy a small amount of power, especially in the winter (we heat on electricity). However, it's designed and sized such that I could add more capacity if needed - I'll have two large arrays of panels up, and will be designing and trenching to add a third, south facing bank, if needed. Use is definitely elastic.
However, as I'm doing the system work myself, extra panel area just isn't that expensive. I'll get (eventually) the 30% back on the system components through the tax refund, and I have some contacts such that I can get pretty good pricing on panels. Mounting materials (wood for the ground mounts and concrete) won't be the cheapest, but they'll be in service for a long time, so no real concerns there.
You aren't taking into account the charging infrastructure you'll need to add, unless you're just planning to use a normal 120V outlet. If you're planning faster charging, that will cost you something and tips the balance away from the Volt.
I'll be charging with 120V - I see no reason to put in higher speed charging. I may run a 240V outlet over to the car eventually (when I put solar in - I'll be trenching all over, what's another trench or two?), but it's quite close to the service box so won't add much real cost, and it's power out towards my shipping container workshop anyway.
The Mazda is fairly fuel efficient, though it's not the more efficient 2L. Assuming gas prices go up as you expect, it will still be in higher demand than vehicles that use a ton of gas. So you can still make the change in the future when you're proven correct. Plug-in hybrids and electric vehicles are likely to continue losing value in that time. Electric vehicles seem to depreciate way faster than gas vehicles for some reason, so it's unlikely holding off is going to hurt you on the trade costs. You might be able to get a newer or nicer example two years down the road for an even trade, though you'll have spent more on fuel in the meantime.
Correct. We're averaging about 31.8mpg (lots of 55mph rural driving), so it's not like it's consuming /that/ much fuel.
I don't know how well the Volt batteries hold up, but I know they have a decently long warranty that you'd be inside of for at least a couple years. Do you know the rate at which people need to replace parts of the electric system and the costs involved? That should be a factor as well, since hybrids have all the normal gas parts and then some.
The Volt batteries are incredibly well done - they're conservative^2, in how they're managed. No concerns there. The likely issue, at some point many tens of thousands of miles in the future, is a stator bearing in the transmission, which is around a $1000 job, but not all of them have the problem, and that's likely 6-8 years off if it happens at the common milage.
Devil's advocacy aside, it seems reasonable. There are more upfront costs involved - being induced to buy more solar capacity, possibly a charger, any registration fees - but those should be paid back over time by reduced gas usage and start making returns at a decent rate if you really do end up saving $650+ per year in fuel. Though remember that the upfront fees don't just need to be made up, but you should compare against what that money could do for you invested instead of spent. You should have the numbers available to you to do that.
The solar capacity... eh, it's an extra 3MWh/yr on a house that uses about 12-14MWh/yr. We've got a pretty deep well, and pure electric heating/cooling/water/etc. I plan to replace some of that load with solar thermal as time goes on, so I expect our electricity use to drop over time as I get those systems online (for hot water, especially). Right now, investment is heavily in our property - I have a non-standard approach in that I'm looking to drive down our expected costs, long term, through productive property improvements (solar, greenhouses, chicken coops, etc) and anti-fragility. So the money saved will go towards things like water storage (which will reduce our power demands for irrigation, among other things) before it would go into traditional investments.
The most efficient way to do it would be to sell the Mazda private party, at which point I think I'd come out with all the fees and such covered - the Mazda, private party, is worth a bit more than the Volt, but there's the convenience factor of having it all done in one transaction, and I'm not exactly sitting on the edge of the financial cliff.