If you're already at the point where you'll be getting a refund the $7500 will not add to that. At best, it can only reduce your liability to zero. It can't give you a positive balance (refund).
I don't think that's quite accurate. Your tax liability is the total amount you "owe" the federal government and it's independent of withholdings or prepayments. The EV credit reduces that amount by
up to $7,500. If your tax liability is less than $7,500, you can only benefit up to your total tax liability. If you're tax liability is only $5,000, you're tax liability will be reduced to zero and you can't take advantage of the remaining $2,500 (and it's not "refunded" to you). I think where people get confused is how withholdings (or estimated tax payments) impact your liability (and the confusion around the word "owe"). In short, any withholdings or prepayments don't affect your liability at all (although they can impact potential penalties depending on your tax situation). At the end of the year, if your withholdings and prepayments are greater than your tax liability, you get a refund. If they're less, you have to pay the remaining balance of your liability. The simplified formula for how much you "owe" the government at the end of the year is:
Due at end of year = (Tax Liability - MIN(Tax Liability, EV Credit)) - Withholdings
If the value is negative, you get a refund.
Imagine these two simplified scenarios:
Scenario A:
Tax liability: $10,000
EV credit: $7,500
Witholdings: $11,000
At the end of the year, you'll get: ($10,000 - $7,500) - $11,000 = -$8,500 (negative means refund to you)
And the government collected $2,500 total from you for the year
Scenario B:
Tax Liability $10,000
EV credit: $7,500
Withholdings: $0
($10,000 - $7,500) - $0 = $2,500 due to the government
And the government collected $2,500 total from you for the year
As you can see, the government gets the same amount of money, regardless of whether you had withholdings or not (although, separately, the government doesn't like to not be paid throughout the year).
The case where you can't benefit from the whole EV credit might look like this:
Tax Liability $5,000
EV credit: $7,500
Withholdings: $6,000
($5,000 - $5,000) - $6,000 = -$6,000 (refund to you)
And the government collected $0 total from you for the year
Whether you get a refund or not at the end of the year doesn't tell you enough about your tax liability to say whether you can use the full EV credit or not. For example, in that last case, you would've gotten a $1,000 refund from the government without the EV credit. With the EV credit, your refund jumped up to $6,000.