Sorry, but the IRS is going to disagree with you.
The fact is that you are required to report the fair market value of all prize winnings to the IRS. It's on you to make a good faith estimate of the fair market value if one is not provided. Tesla also shows the value of your prize as a credit against your contract price due when you claim your referral prize together as part of a larger system. (It's on my contract).
Since this is a retail product that is sold in "stores", the current retail market price (as noted above) is what the IRS is going to look at if they audit you, and no, the IRS is not going to buy an argument that there's zero value in the red powerwall vs. the retail white one that gets sold for $8800.
Just report it on your taxes in the year that the system installation is completed and be safe. There's a chance that the IRS is not going to pursue everyone that got powerwalls, especially if they levy and collect a hefty fine from Tesla for noncompliance, but it's on you if you want to take that risk. Even if the 3 year statute of limitations passes before the IRS catches up, they could still argue fraud (for which there's no statute) or if the value of the prize causes a > 25% error in your tax due (possible if you have little taxable income, i.e. nontaxable retirement income).
Also, don't forget that many states follow the federal income rules, so your state will also come after you as well (assuming you are resident in an individual income tax state).