If the total system costs $29,000 and you get a $7,800 rebate from the utility company (I strongly recommend contacting your electric company to verify that amount, rather than just relying on the salesman), that puts your system cost at $21,200.
That means that your potential Federal Credit is $6360 ($21,900 x 30%). If South Carolina works the same way, that would make the long-term South Carolina credit $5300 (if South Carolina uses the pre-rebate amount, it would be a $7250, but I suspect it would be the same was as Federal, but I'm unsure about that).
As the others pointed out above, your Federal Credit will be limited, but the unused amount will carry forward to future years until it can be used. The amount you can use each year will be limited to an amount somewhere between the amount on Line 47 and Line 56 of your Form 1040 (it depends on which other credits you have on Line 48-54).
You can look at prior tax returns to get an idea of those amounts, but 2018 taxes have changed quite a bit, so it is likely your 2018 Line 47 and Line 56 will be slightly lower than previous years.
So that amount will directly increase the amount of your 'normal' refund by that credit amount. So if you would otherwise get a $1000 refund (without any solar credit) and you are allowed a $4000 solar credit (based on Lines 47 and 56), then you will receive a total of $5000 for your Federal refund.
The credit for South Carolina is also limited each year. It is limited to the lower of (1) 50% of Line 10 of your South Carolina return (if you have other credits, it could possibly be less than that) and (2) $3500. Again, the unused amount will be carried forward to future years until it can be used (up to 10 years).
