Congrats! I would try to get an install day before the new year. The tax credit drops from 26% to 22% on 1/1. I would call Tesla to see what they can do. (I called and got them to move it up by about a month.) As long as the installation starts in 2020, you're eligible for the 26%.
The "construction starts" safe harbor applies only to commercial systems that are eligible for the Investment Tax Credit under section 48 of the Tax Code. Rooftop solar on a home is eligible only for the Residential Energy Credit under section 25D and that stipulates the system must have been "placed in service" during the taxable year.
While there is an
IRS Letter that goes into detail about the definition of "beginning of construction" it is in reference only to section 48. Furthermore, the IRS includes a note in the
instructions for the form to claim the
Residential Energy Credit that guidance for section 48 (including that notice) is inapplicable.
So, to get the 26% credit for the 2020 tax year you better have the system up and running before the end of year.