Demand Charge

What a Demand Charge Is

A demand charge on an electricity bill is a fee based on the highest rate at which a facility draws power from the grid during a billing period. Unlike energy charges, which depend on the total electricity consumed over time in kilowatt hours, demand charges are based on peak power demand measured in kilowatts.

A demand charge does not price the electricity a facility uses. It prices the capacity the system has to hold ready to serve that facility's highest moment. The energy charge answers how much was consumed over the month, in kilowatt hours. The demand charge answers how much the grid had to be built to deliver at once, in kilowatts.

Utilities include demand charges to recover part of the cost of maintaining electrical infrastructure capable of serving a customer's maximum load. Substations, transformers, feeders, and other distribution equipment must be sized to deliver the highest level of power a facility might require at any moment, even if that level is reached only briefly.

Because this infrastructure must always be available, commercial customers are billed not only for the electricity they consume but also for the maximum power level their operations require during the billing cycle.

How Utilities Calculate Demand Charges

Demand charges are calculated using interval demand measurements recorded by the electric meter. Most commercial meters measure average power demand over 15 minute intervals.

At the end of the billing cycle, the utility identifies the interval with the highest average demand and uses that value as the billing demand.

The basic calculation is:

Demand Charge = Peak Interval Demand (kW) x Demand Rate ($ per kW)

In many tariffs, customers served at primary voltage may face lower demand rates than customers served at secondary voltage because primary service customers require less distribution infrastructure and fewer transformation stages. That discount is not free, and whether the trade is worth taking is a real decision for a growing account: see is ComEd primary voltage service worth it.

The determinant also varies by utility, and it varies more than most owners expect. Two questions decide most of the economics: which hours are eligible to set the charge, and whether a prior month's peak carries forward. Worked through for the two Illinois utilities and for a deregulated state next door: Ameren Illinois, and Ohio commercial accounts.

If you are on ComEd, you can skip the arithmetic: our demand charge calculator applies the exact rate for your delivery class from ComEd's tariff, including the per-rider rounding a simple multiplication misses.

Why a Single Peak Can Set the Monthly Demand Charge

To understand how demand charges work in practice, it helps to follow the trail from the utility bill back to the underlying meter data. A commercial electricity bill reports a single billed demand value, and that one number sets the demand charge for the entire billing period. The rest of the analysis involves identifying where that value came from in the facility's interval meter data.

Electric utilities determine demand charges using interval meters that record the facility's load throughout the billing cycle. Examining the interval data locates the exact moment the facility reached its highest recorded demand during the billing period. That single interval becomes the data point used to determine the billing demand.

Plotted across the full billing period, the pattern becomes clear. The facility operates with a repeating daily load cycle, but one short spike rises above the rest of the load profile. Demand charges are determined by this peak interval rather than the average load across the month. Even though the facility spends most of its time operating at much lower levels, that brief spike establishes the demand level used to calculate the monthly demand charge.

You don't pay for how much electricity you use. You pay for the single moment you use the most.

A Reconciled Demand Charge

A worked example makes the trail concrete. A single meter commercial site on ComEd's Small Load class was billed for 22.92 kilowatts of demand in one fall billing period. Reconstructed from the site's thirty minute interval data, the highest on-peak interval measured 22.92 kilowatts, matching the billed demand exactly, and it fell at 5:30 PM, inside the 9:00 AM to 6:00 PM window. The recomputed distribution demand charge came to within a few cents of the bill. No off-peak interval in the period rose above it, so the billed demand and the physical peak of the period were the same moment. This is the ordinary case: one interval, located in the data, reconciled against the bill.

The free demand charge tool runs this reconstruction on any ComEd commercial account and shows the exact interval that set the charge.

When the Physical Peak Isn't the Billed Peak

The clean case above, where the physical peak and the billed peak are the same interval, is not the only one. Sometimes the highest level the meter records is not the level that sets the charge.

A two meter commercial site on ComEd's Small Load delivery class drew a site coincident peak of 40.91 kilowatts during one summer billing period. That value, measured across both meters at the same thirty minute interval, matched the billed demand on the delivery portion of the bill exactly, and it produced a Distribution Facilities Charge of $596.88 for the month.

The interval that set the charge began at 9:00 AM. Thirty minutes earlier, at 8:30 AM, the same site had drawn 41.94 kilowatts, a materially higher level of demand. That earlier interval was not billed, because ComEd measures billable demand only during the on-peak window of 9:00 AM to 6:00 PM on weekdays. The physical peak of the day sat just outside the window. The billed peak was the first qualifying interval inside it.

This is what makes demand charges difficult to reason about from the bill alone. The billed number is not the highest level the meter recorded. It is the highest level recorded inside a specific tariff defined window, reconciled against the bill. Reading it back requires the interval meter data and the tariff rule examined together.

What Produced the Billed Interval

Once the billed demand value is identified, the next step is determining what produced the peak demand interval that set the charge.

If a true coincident peak occurred, one or more intervals in the meter data will show demand materially higher than the facility's typical operating level. This indicates that equipment loads overlapped or ramped simultaneously during a short period of time.

If tariff rules are dominant, the billed demand may correspond to defined peak windows or seasonal demand definitions rather than to unusual facility behavior. In these cases the charge reflects how the tariff defines billable demand rather than an isolated operational event.

If interval aggregation is involved, short duration spikes may be partially hidden within the meter's recording interval. Demand may appear relatively stable in averaged data even though brief spikes occurred inside the measurement window.

Distinguishing between these explanations requires three pieces of information: the billed demand value, the interval definition used by the tariff, and the interval level demand data recorded by the meter. Once those elements are examined together, the source of the demand charge can usually be traced to the specific interval that established the peak demand for the billing cycle. For worked examples, we traced that interval on six real ComEd commercial bills, reconciled to the meter.

Where Demand Charges Appear on a Commercial Electricity Bill

Demand charges typically appear in the delivery or distribution section of a commercial electricity bill. They are usually listed as a separate line item showing the billing demand in kilowatts, the demand rate in dollars per kilowatt, and the resulting charge.

Common labels include demand charge, billing demand, peak demand, or distribution demand.

Why Demand Charges Are Often Higher Than Expected

Demand charges often surprise facility operators because they are driven by short operational events rather than total energy usage. When multiple systems operate simultaneously, their combined load can produce a peak demand significantly higher than normal operating levels.

Without access to interval meter data, many businesses cannot easily identify when these peaks occur. As a result, a short spike lasting only a few minutes can determine a large portion of the monthly electricity bill.