The Residential Energy Credit or solar panels (I assume that this is what you mean) is - for 2020 - 26% of the qualified expenses for the solar panels. It was 30% for tax year 2019, 26% for tax year 2020 and 22% for tax year 2021. After that, the credit may expire, depending on what Congress does. See https://www.irs.gov/newsroom/energy-incentives-for-individuals-residential-property-updated-question...
The credit is a non-refundable credit. That is, it can reduce your tax to zero, but not below that.
In your case, let's say you have your tax assessed at $1,000 (totally made up number). You normally withhold more than $1,000, let's say $1,500, so your refund would normally be $500.
If you got the residential energy credit for, say, 26% of $10,000 in qualified spending, that would be $2,600 in a credit. Your tax would be reduced from $1,000 to $0, and $1,500 of your withholding (i.e., all of it) would be refunded to you.
Any credit that was not used ($1,600) would be carried over to next year. So you could apply the carryover to the next year under the same rules.
NOTE: it is not yet known if Congress will allow the carryovers past 2021.
Clear?
