@Veerstraat
The actual tax law section 25D, defines solar energy expenditures this way.
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.
The tax code doesn’t say anything about limiting the credit to the first installation, or only allowing the credit in a single year. There is nothing in the law that would disallow the credit for somebody who is expanding their system or replacing an outdated and inefficient system with a new one.
