While I'm not an accountant, your logic as to the ITC seems sound. I'm of the same opinion. That said, if I rightly recall, there are posts upthread which assert that it's possible to claim the ITC based on the full cost while simultaneously receiving the full SGIP rebate.
I agree that if the ITC isn't worth a great deal, then it's probably not worth claiming. With no ITC requirements to worry about, we could charge the Powerwalls from the grid during "super off peak" hours, if supported by Tesla, and get the maximum possible economic benefit from TOU load shifting and solar PV NEM credits. Skipping the ITC could also give us the freedom to "top off" the Powerwalls from the grid whenever there's a big snowstorm up here in the mountains that threatens to simultaneously knock out our grid power and bury our solar panels. (That said, "snow" seems like a foreign concept right now, as it's already December and we haven't seen a single flake yet.)
With continued growth in solar PV deployments, I expect that the late morning hours will eventually have to be designated as "super off peak", at least during Spring. That would push homeowners like us to charge our batteries during those hours instead of exporting our solar production.
My "Self-Generation Incentive Program Residential Energy Storage Eligibility Affidavit" document which I signed included a line "Taking Federal Investment Tax Credits (ITC): No.
As an affidavit, I think that if you then take the ITC, you could be found at fault (if caught).
Proposed System Information Equipment Technology: Electrochemical Storage System
Manufacturer: Tesla
System Model: 110856700C
Other selfgeneration or storage equipment onsite?
Charged at least 75% from renewables? Yes
Total Rated Capacity (kW): 10
Total Energy Storage Capacity (kWh): 27
Discharge Hours Duration: 2.7
Project Finance Total Eligible Project Cost (TEPC): $15,406.75
Ineligible Project Cost:
Taking Federal Investment Tax Credits (ITC): No
ITC as a % of TEPC: %
Approved California Manufacturer Equipment: false
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Also, it seems there are some people planning to charge their batteries at night when power costs are low and discharge it during the day which is not allowed to claim the 30% ITC. That is why the IRS instituted the 75% rule. An article at gosolar.com seems to confirm this.
"Additionally, what’s labeled by the IRS as "dual use property" is the ability to charge the battery from the grid
when rates are low, and discharge when rates are high. According to the IRS this is not what the 30% ITC was
meant for; the ITC was meant to facilitate the adoption of clean energy (such as solar), and in the case of
batteries to store it for later use. Not to arbitrage utility rates (buy low, sell high). According to Renewable
Energy World Magazine there is a so-called "75% Cliff" whereby if the consumer charges the battery with less
than 75% renewable energy, then the battery is not eligible for the tax credit. Above 75% the battery system is
eligible for the ITC based on the percentage of renewable energy that is charging the battery system. So if
you charge your battery with 90% solar electricity and 10% from the grid, then you are eligible to claim 90% of
your 30% ITC for the cost of those storage components. This ruling applies over the first five year period of
ownership, after which point dual use charging would not affect the tax credit rate..."
