Commercial Demand Charges in Illinois: What They Are & How to Manage Them

For many Illinois commercial accounts, the demand charge — not the per-kWh supply rate — is where cost concentrates. It bills how hard a facility pulls at its peak, and it responds to different levers than a supplier contract. Understanding it is often the highest-value part of a commercial energy review.

Who this is for

Facilities with demand-billed rate classes and uneven load — manufacturing, warehousing and cold storage, and any operation with large motors, compressors, chillers, or simultaneous equipment starts. If a bill shows a "kW" or "demand" line, this applies.

How demand billing works

The utility measures peak demand (in kW) over a short interval, commonly 15 minutes, during the billing period, and charges for that peak separately from energy. Because it is driven by coincident equipment use, two facilities with identical monthly kWh can face very different demand charges. That is why total energy alone is a poor predictor of a commercial bill.

The distinction is between how much energy you use and how fast you use it. Energy (kWh) is the total volume over the month. Demand (kW) is the rate of draw at the single worst moment. A shop that runs one large machine at a time draws less peak power than one that starts several at once, even if both consume the same kWh by month's end. The demand charge prices that instantaneous rate — and because it captures the highest interval, a peak that lasts only fifteen minutes can set the charge for the entire billing period.

A worked example, without invented numbers

Picture two warehouses that use the same total kWh in a month. The first staggers its equipment: forklift chargers cycle overnight, the compressor runs on a schedule, and the HVAC ramps gradually. Its load is relatively flat, so its peak kW stays modest. The second starts everything together at the beginning of each shift — chargers, compressors, and HVAC all pulling at once. That simultaneous start creates a tall, brief spike. Even though the monthly energy is identical, the second warehouse registers a much higher peak, and the demand charge follows the peak. The lesson is directional, not a promise: where peaks come from coincident starts, spreading those starts out is the lever that touches the demand line. Whether it can be done, and by how much, depends entirely on how the facility actually has to operate.

Demand vs. supply — two different levers

Shopping a supplier changes the supply rate. It does not, by itself, change the utility's delivery demand charge. Managing demand — understanding when peaks occur and whether they can be shifted or flattened — is a separate exercise from procurement, and for peaky loads it can move the bill more than a supply-rate change. A complete review looks at both.

Capacity tags carry demand into future supply cost

In the PJM region that includes ComEd, an account's capacity tag (PLC) is derived from its usage during system peak periods and feeds the capacity component of supply pricing for a delivery year. Peak behavior in a few key hours can therefore affect cost well beyond the current month's demand line. Read the PJM capacity and Illinois business bills guide for how delivery years and PLC interact — and note that a PJM auction clearing price is not a bill rate.

Where demand peaks tend to come from

For demand-billed operations, peaks usually trace back to a handful of recurring causes:

• Simultaneous equipment starts — several large motors, compressors, or chillers energizing at the same time, often at shift change.
• Large individual loads — a single big motor or process that, when it runs, dominates the facility's draw.
• Cooling on hot afternoons — HVAC and refrigeration ramping together during summer peaks.
• Startup inrush — motors and compressors drawing more at the instant they start than while running.

None of these is visible on a monthly summary; they show up only in interval data, which records the shape of the load through the day. That is why interval data is the working material of a demand review: it turns "the bill is high" into "here is the interval that set the peak, and here is what was running."

What to watch for

• Ratchet-style provisions. Some rate structures carry a peak forward so that a single high month influences later billing. Read the rate class rules rather than assuming this month's peak stays in this month.
• Capacity tag timing. Because the PLC is derived from usage during system peak periods, behavior in a few key hours can affect the capacity component of supply cost for a whole delivery year — long after the hours themselves.
• Chasing supply while ignoring demand. For a peaky load, the demand and capacity levers can matter more than the supply rate. Shopping a supplier alone leaves the largest driver untouched.
• Confidential data. Interval files and bills carry account identifiers; handle and share them securely.

What a demand review needs

Monthly bills establish the demand pattern; interval data is what reveals when peaks occur and whether they are addressable. Reconcile interval data to billed kWh and kW first, then look for recurring peak drivers. Pair the finding with the PJM capacity guide to see how a lower peak can carry into future supply cost through the capacity tag. Nothing here promises a specific reduction — demand outcomes depend on how a facility actually operates, and any change has to fit the operation first.