How do I get the 15% Massachusetts tax credit for installing solar panels using TurboTax?

Unfortunately, you'll need to print and mail your Massachusetts state return to claim this credit.  To do this,

1. You'll need to complete Schedule EC yourself.  You can download and print it from Schedule EC, Solar and Wind Energy Credit.
2. Once you've completed Schedule EC, you'll need to enter the credit amount, within TurboTax, on the MA Credits screen as a "Business related non-refundable credit."  Disregard that the fact that you're not a business, you just need to get this data into TurboTax.
3. Select "Solar and Wind Energy Credit."  and enter any relevant amounts.
4. TurboTax will generate Schedule CMS for you.
5. When your tax return is complete, print it and mail, since you're not able to eFile with the energy credit form.

Thanks for your quick answer. Do you have insight into the expiration date (which is a required field)?

According to Schedule CMS instructions at <a rel="nofollow" target="_blank" href="https://www.mass.gov/files/documents/2019/02/12/dor-2018-inc-sch-cms.pdf">https://www.mass.gov/files/documents/2019/02/12/dor-2018-inc-sch-cms.pdf</a>, it's the tax period end date in which the credit originated -- assuming you're filing your 2018 return, this'd be 12/31/2018.

Thank you!