This is similar to an academic concern that I've been wrestling with in my mind regarding my own personal situation. Maybe those who are more tax savvy than me can sort this out.
Being retired I'm precisely in the situation that you describe, that is I do not have enough income to generate a $7,500 tax liability. So inorder to take advantage of the full tax credit I have to generate additional tax liabilities. I can do this by withdrawing from my 401K. By withdrawing the balance owed on my Model S, plus an additional allowance to deal with the increased tax burden of moving into a higher tax bracket generated by the additional income, I can create the necessary tax liability to take the full credit. So the question to the tax experts is, "Have I really saved anything by taking advantage of the full tax credit if I have to increase my tax liability to get it?"
Make a spreadsheet.
You run two or more scenarios (or more, as necessary), including appropriate assumed rates of interest in the 401k and outside of the 401k, and figure out your total after-tax return when you're done. It's a simple spreadsheet problem. Don't forget state taxes. You may also need to consider the AMT and the preferential rates on capital gains and dividends -- and the projected *future* status of those rates.
I find that *usually* you benefit if you can take money out of a retirement account, not pay federal taxes while doing so, and reinvest it. Unless the retirement account offers funds with a much better return than you can get outside the retirement account...
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Can an unmarried couple living together but filing as individuals claim a percentage of the credit each?
You can buy the car jointly in one of two different ways: as tenants in common or as joint tenants with right of survivorship.
If you buy it as tenants in common, you each own a percentage of the car. If you paid half and the other person paid half, you literally each own half a car.
I do not know if there are special rules for this tax credit, and you had better get expert tax advice. If it works like most tax credits, you would then each get half of the tax credit. If it worked like some rarer tax credits, one of you would have to take the whole tax credit and the other one would have to agree to it. If it worked like yet other tax credits, both of you could get the entire credit. If it worked like yet other credits, neither of you would get a credit. It's also possible that you would be construed to each be buying a car which cost half the price of the Tesla, in which case the tax credit (since it's based on value) might be computed to be a number based on this smaller value..
If you own it jointly with right of survivorship, the *tax laws* generally treat it the same way as owning it as tenants in common -- they look at who contributed the money to buy it and consider you to own it by percentages. However, the *legal rules* (inheritance and so on) are completely different, which could have a further effect on whether you "qualify" as having bought an electric car.
In short, you need advice from someone who really knows their stuff.