You can't defer the credit.
You claim the credit in the year you install the solar equipment. The dollar amount of the credit is based on the contract price for the solar system, even if you are paying over time. The credit is added to your tax refund, and depending on your tax situation, some of the credit may be carried forward to the next year. What you do with the money after that is up to you. But you can't claim the credit a little bit at a time.
