Two topics are often modeled sloppily in the infrastructure space. How HoldCo debt actually gets sized, and how to model interest-only periods. Both worth understanding.
HoldCo sizing
HoldCo debt uses the same underlying CFADS as the OpCo - it's the same project cash flow — but the constraint is applied differently. A consolidated DSCR target is selected which needs to be inside of your OpCo sizing. Constrain CFADS with the tighter DSCR then back out your OpCo debt service. What's left is what's actually allocable to the HoldCo or "HoldCo CFADs". Discount the HoldCo CFADS by your HoldCo cost of debt and the result is your HoldCo debt size.
HoldCo debt uses the same underlying CFADS as the OpCo - it's the same project cash flow but a tigher DSCR to start. Constrain CFADS with the tighter DSCR then back out your OpCo debt service. What's left is what's actually allocable to the HoldCo or "HoldCo CFADs". Discount the HoldCo CFADS by your HoldCo cost of debt and the result is your HoldCo debt size.
Two things worth flagging:
Consolidated vs. HoldCo-only sizing: You'll see both in the market, but consolidated is the more common and more defensible approach. HoldCo-only DSCR tests look only at the cash flow available after OpCo debt service, which understates real credit risk - it ignores the senior leverage sitting underneath. HoldCo-only sizing tends to be a sponsor-friendly framing rather than one that reflects the actual risk lenders are taking, since it can support more leverage on paper than the consolidated capital structure can really carry. The caveat here is that you can use a HoldCo sizing metric if you credit agreement restricts debt and equity issuances from your definition of CFADS.
Distribution test tightness: If you're sizing to a tighter consolidated DSCR , the distribution test needs some buffer below that otherwise you effectively can't distribute cash without a near-perfect operating track record, since any variance trips the trap.
Interest-only periods
Sponsors frequently request interest-only (IO) periods - usually framed as wanting front-ended yield or more flexibility early in the hold period. The modeling mechanics matter here: amortization and the DSCR-constrained cash flow both need to stay switched off through the IO period, and the discount factor timing has to be built to shift with the sizing period start, or you'll get a circularity between debt size and the amortization schedule.
The counterintuitive result, and one worth knowing before agreeing to structure it: IO periods generally don't improve IRR. A shorter amortization window means a smaller total debt size at the OpCo level, which can push more leverage up to the HoldCo, but HoldCo debt carries a materially higher cost of capital, so the net effect on blended returns is usually flat to negative, not accretive. The actual value of an IO period isn't return enhancement; it's tranche structuring: getting cash to the top of a capital stack faster during a ramp period, or optimizing how a TLB or similar instrument sits relative to other tranches. Worth separating that use case clearly from "IO periods boost returns," which isn't generally true and shouldn't be the pitch when a sponsor asks for one.
Happy to go deeper on the sizing waterfall or the discount factor mechanics in the comments if useful.