Yes, it is taxable. If you elect to avoid paying full taxes on this income, it might be cleaner to declare a separate "business" to sell electricity with your passive participation. You should then be able to claim solar panels and batteries as a partial asset of that business. (Partial because it is also used for your personal matters). Since the utility companies send you the details of how much electricity was sold versus how much was used for your house, it is quite simple to calculate the percentage of the business use.
Please note that calculation of depreciation is a little bit tricky because it allows you to choose multiple options. Besides that, according to IRS, the value of the asset should be decreased by 50% of the amount you received as the tax incentives credit. For example, if your spent $30K is the total cost for the panels and installation etc., and received $8K of the tax credit, the value of the asset used to calculate depreciation should be $30K-0.5*$8K.
Disclaimer: by no means consider this as legal advice. I am not an accountant, I am just a person who fed up with multiple contradictive explanations by different accountants, and decided to read IRS documents by myself. And this is what I plan to use. However, because I am not a professional accountant, there is a chance I'm missing something.
