Why the Whole Framework Runs on a Signature
Notice 2026-15 gives taxpayers three interim safe harbor methods for calculating the material assistance cost ratio, along with the option to calculate it under general rules. What all of those paths share is a dependence on supplier documentation. There's no IRS-issued form for this calculation, and no independent government registry a developer can check instead. The primary evidence a taxpayer has to support its FEOC position is a self-certification letter from each tier-one supplier, signed under penalties of perjury, confirming FEOC status and, where relevant, the cost breakdown needed for the ratio calculation.
That structure puts a significant amount of leverage in the hands of manufacturers who may have little incentive to cooperate. A supplier being asked to certify facts under penalty of perjury, particularly facts touching on its own ownership structure or its upstream sourcing, is being asked to take on legal exposure for someone else's tax position. Some suppliers are simply declining.
What Happens When a Supplier Refuses
When a certification refusal happens, it tends to happen late, often after a developer has already committed to a supplier relationship and structured procurement timelines around it. At that point, the options are limited and none of them are fast. A developer can attempt to source an equivalent component from a supplier willing to certify, which may mean absorbing new lead times in a market where, as we've covered before, panel and equipment lead times are already stretched. A developer can attempt to independently verify FEOC status through other documentation, though that path is significantly more resource-intensive and less certain to satisfy a buyer's diligence standard. Or, in the worst case, a developer proceeds without adequate certification and accepts the compliance risk that creates, which as we described in The 2026 Supply Chain Trap: Surviving FEOC Limits and the Direct Pay "Haircut" Under Section 48E, means risking full credit elimination rather than a reduction.
Why Buyers Are Treating This as a Screening Question, Not a Closing Detail
Buyers have caught on quickly. Transfer agreements now routinely include explicit representations covering supplier certification accuracy, and buyers are asking to see the actual certification letters, not just a summary confirming they exist, earlier in the diligence process than they used to. This is part of the same shift toward pricing FEOC-related uncertainty directly that we described in Why Public Companies Can't Actually Certify Their Own FEOC Status: buyers no longer take a compliance representation at face value, they want to see the documentation underneath it, and a missing or incomplete supplier certification is now a red flag that gets surfaced early rather than discovered at closing. Insurers are reacting the same way. As we discussed in The Insurance Policy Quietly Holding the FEOC Market Together, underwriters are increasingly asking for the same certification package a buyer would want to see before they'll price a policy at all, which means a missing signature can stall insurance coverage as easily as it stalls the underlying deal.
What Developers Should Do Before It Becomes a Problem
The fix here has to happen upstream of procurement, not after a supplier relationship is locked in. Developers should be requesting certification commitments as part of the supplier negotiation itself, before purchase orders are finalized, and should be prepared to walk away from suppliers unwilling to provide one, even at a cost premium elsewhere. Given how tightly this risk connects to broader FEOC exposure, it's also worth having tax counsel review certification language before it's requested, since a signed letter that doesn't match the Notice 2026-15 format may not hold up as adequate documentation even if the supplier is willing to cooperate. Treating supplier certification as a procurement checkpoint, rather than a paperwork step handled after the fact, is the difference between catching this problem in week two of a project and discovering it in month eight, when the options for fixing it have narrowed considerably.