A repair restores or maintains the system to as-is or as-was condition. Repairs are the ordinary responsibility of every property owner to keep their own stuff in good condition and there are no general or special solar tax breaks for repairs.
An improvement adds value to the system or extends it's useful life. Fixing broken connections would be a repair. Replacing the inverter or batteries because they were worn out or inefficient is probably an improvement because it extends the life of the system.
As I read the instructions for form 5695 and the actual law, I see the solar property credit is for improvements. It does not specify new installations only. That means you can include the cost of improvements (but not repairs) as a tax credit on the Residential Energy Efficient Property Credit section of the deductions and credits page. If audited, you will need to be able to show how you determined that your costs were improvements or repairs.
You can also add the cost of the improvement to your cost basis for the home, which may reduce your capital gains when you sell. However, you must reduce the cost by any federal, state or local credits that you get.
References:
"If you made energy saving improvements to more than one home that you used as a residence during 2021, enter the total of those costs on the applicable line(s) of one Form 5695."
https://www.irs.gov/pub/irs-pdf/i5695.pdf
"The term “qualified solar electric property expenditure” means an expenditure for property which uses solar energy to generate electricity for use in a dwelling unit located in the United States and used as a residence by the taxpayer."
https://www.law.cornell.edu/uscode/text/26/25D
