Right - the client didn't do a very good job of characterizing the scenario. He said "Tesla" which is actually correct, but for some that can be misleading because we always think "Oh yes, Tesla cars. It actually is a Tesla Powerwall. He told me that fact after my original post above (gotta love the taxpayer). 

And yes, a Tesla Powerwall is used in connection with solar panels. The Tesla Powerwall is described as a battery system that complements a solar panel installation or adds to an existing solar array.

As long as all of the energy used by qualified solar electric property is derived from the sun then it qualifies for the solar energy credit.

A “qualified solar electric property expenditure” is an expenditure for property that uses solar energy to generate electricity for use in a dwelling unit. The dwelling unit must be located in the U.S. and used as a residence by the taxpayer.1 1Code Sec. 25D(d)(2)

Fun stuff.