Fixed vs. Index Commercial Electricity Contracts in Illinois

Choosing between a fixed and an index electricity contract is one of the most common decisions an Illinois business makes at renewal — and it is often framed too simply. Neither is universally cheaper; they carry different risks, and the right fit depends on the account.

Before comparing the structures, it helps to remember what all of them have in common. Each is a way of pricing the supply portion of the bill only — the electricity itself. None of them changes delivery, metering, most utility riders, or taxes, which stay with the utility no matter how supply is priced. So the fixed-versus-index choice is a choice about how to manage risk on one slice of the bill, not the whole thing. That framing keeps expectations realistic: even a perfectly fixed supply price does not fix the total bill. For the broader mechanics of supply versus delivery, see how commercial electricity choice works in Illinois.

Fixed contracts

A fixed contract sets the supply price per kWh for the term. Its value is budget certainty for the components the contract includes. The caution: a low headline fixed rate can carry broad exclusions, and delivery charges, riders, taxes, and any pass-throughs still move. Confirm what is included before treating a fixed price as "locked."

The mechanic behind a fixed price is that the supplier hedges the expected volume forward, buying the power to serve your contract ahead of time so it can quote one number for the term. That hedge is what you are paying for: the supplier absorbs the market's month-to-month movement so you do not feel it. Two consequences follow. First, a fixed price usually sits above the average the market might deliver, because the supplier prices in the cost and risk of the hedge — you are buying certainty, not the lowest possible average. Second, the completeness of the fix depends entirely on which components are inside the fixed price. A price that fixes energy but passes through capacity and transmission is only partially fixed, and the parts left floating are exactly where a budget surprise can appear. The most important question about any fixed offer is therefore not "how low is the rate" but "what does this rate include, and what still moves."

Index (pass-through) contracts

An index contract lets supply track a market index, so the price can rise or fall month to month. It offers potential savings and transparency with more variability — appropriate for accounts that can absorb monthly swings or that want to stay exposed to falling markets. It requires more attention than a fixed contract.

Under an index product you are effectively holding the market risk yourself instead of paying a supplier to hold it. When the market falls, you feel it quickly; when it rises, you feel that quickly too. The supplier's margin is usually a defined adder over the index, which is why index contracts are often praised for transparency — you can see the index and the adder separately. The trade-off is administrative and psychological: someone has to watch the monthly cost, explain the swings to a budget owner, and decide whether and when to lock a portion. An account with steady cash flow and the tolerance to ride out a bad month can capture the benefit; an account that would be disrupted by a single high month usually cannot, regardless of the long-run average. Reading the direction and volatility of the market matters more here than with a fixed price — see how to read Illinois commercial energy price trends.

Block-and-index (layered) contracts

A block-and-index product fixes a portion of expected volume and floats the rest. Larger loads use it to balance certainty and flexibility and to hedge in stages. It is more complex to administer and compare.

The idea is to split the load into a fixed "block" that behaves like a fixed contract and a remainder that behaves like an index contract. A business that fixes, say, the predictable base load and floats the variable portion gets budget certainty on the part it can forecast while keeping exposure — up and down — on the part it cannot. It also allows layering: locking blocks in stages over time rather than committing the whole volume at one market moment, which spreads out the risk of signing on a single bad day. The cost of that flexibility is complexity. There are more moving parts to administer, more terms to read, and the offers are harder to compare side by side because two suppliers may block and float different proportions. This structure rewards accounts that have the size, the internal attention, and the data (interval usage, a real load forecast) to manage it.

How to choose

The decision comes down to the account's risk tolerance, load shape, and budget cycle:

• Tight budget, low appetite for variability → fixed often fits.
• Ability to absorb monthly swings, view on the market → index may fit.
• Large, sophisticated load → block-and-index can balance both.

Whatever the structure, compare offers on a fully-loaded, matched basis — see how to compare offers apples-to-apples — and read the contract terms, not just the rate, per the contract review guide. For the broader picture, start with commercial electricity in Illinois and the full commercial energy procurement framework.

Common mistakes when choosing a structure

• Judging a structure by its name. "Fixed" is not automatically safe and "index" is not automatically cheap. What matters is which components each price includes and how the excluded ones behave.
• Comparing a fixed rate against an index rate directly. A fixed price and a current index price answer different questions — certainty versus a snapshot. Comparing them head to head as if they were the same number invites a bad decision.
• Ignoring the exclusions. A low fixed rate with capacity and transmission passed through is only partially fixed; a "transparent" index with a large adder may not be as cheap as it looks. Read what floats.
• Choosing index without an owner. Index and block-and-index products need someone watching the market and the monthly cost. Without that attention, the flexibility becomes unmanaged risk.
• Matching structure to the market instead of the account. The right structure follows the account's budget tolerance, load, and cycle — not a hunch about where prices are heading.

A short checklist before signing

Confirm the exact term and delivery dates. List every component the price includes and every one it passes through. For fixed offers, ask what triggers a pass-through or change-in-law adjustment. For index offers, identify the specific index and the adder, and confirm whether any portion can be locked later. For block-and-index, confirm the fixed proportion and how the floating remainder is priced. Check bandwidth or volume-tolerance terms and early-termination language. Only then compare offers on a matched, fully-loaded basis — and time the decision against the account's renewal window.

Sources

• Illinois Commerce Commission — Electric Choice Basics
• Illinois Power Agency — Electricity Supply Rates

No contract structure guarantees savings; each is a risk choice for a specific account.