What Is a Capacity Tag (PLC) and How It Is Set

If you manage electricity costs for a commercial or industrial site in Illinois, you will eventually run into the term "capacity tag," also called peak load contribution, or PLC. It is one of the less visible drivers of a business electricity bill, yet it can influence a meaningful share of what you pay. This article explains what a capacity tag is, how it is measured, and how it moves from a grid calculation onto your account.

What a Capacity Tag Represents

Regional grid operators must make sure there is enough generating capacity available to serve everyone at the moment demand is highest. To pay for that standby capacity, the grid assigns each customer a share of the total peak demand. Your capacity tag is that share, expressed as a quantity of demand attributed to your account.

Think of it as a measure of how much your facility contributes to the grid's busiest moments, rather than how much energy you use over a month. Two businesses can consume the same total kilowatt-hours over a year and still carry very different capacity tags, because the tag depends on when you were drawing power, not just how much you drew overall.

In northern Illinois, ComEd operates within the PJM Interconnection, so capacity tags there follow PJM's rules. In central and southern Illinois, Ameren Illinois operates within MISO, which uses a related but distinct approach. This article focuses on the PJM framework that applies to ComEd customers, while noting the MISO difference where it matters.

How the Tag Is Measured

The core idea is straightforward: the grid identifies the hours when total system demand was at its highest during the prior year, and then it looks at how much power your facility was pulling during exactly those hours.

In PJM, the relevant reference points are the system coincident peak hours. "Coincident" means the measurement is tied to the grid's peak, not your individual peak. If the whole PJM footprint hit its highest demand on a hot summer afternoon, your usage during those specific hours is what counts toward your tag, even if your own busiest hour happened on a different day.

Because interval meters record usage in short increments, the utility can look back at your consumption during those defined peak hours and average it to produce your contribution figure. That figure, adjusted by loss and reconciliation factors defined in the operator's rules, becomes your capacity tag for the upcoming delivery year.

A few practical implications follow from this design:

• The measurement window is in the past. By the time you receive a tag, the hours that set it have already happened, so it reflects decisions you already made.
• The tag is largely fixed for the delivery year. Once assigned, it does not float with your monthly usage. It resets only when the next annual measurement produces a new value.
• Timing matters more than volume. Reducing usage during the specific peak hours has a different effect than simply using less energy across the whole month.

Understanding this timing is the foundation for any effort to manage capacity costs, a topic covered in more depth in our guide on capacity tag forecasting methods.

Who Sets the Tag, and Who Does Not

It is important to be clear about roles, because the Illinois market separates delivery from supply.

Your delivery utility, ComEd or Ameren Illinois, always owns the wires, the meter, outage response, and the delivery charges on your bill. Working within the grid operator's rules, the utility is the party that measures your interval usage and calculates the capacity tag. The grid operator, PJM or MISO, defines the methodology and the peak hours.

A competitive supplier, known in Illinois as an alternative retail electric supplier (ARES), affects only the supply portion of your bill. A supplier does not create or change your capacity tag. Instead, it takes the tag that the utility and grid operator have already assigned and uses it as an input when pricing your supply. That is why two suppliers can quote the same customer differently: they may treat the capacity component in their pricing in different ways, even though the underlying tag is identical.

If you are evaluating supply options, understanding how the tag flows into a quote is useful background. Our overview of commercial electricity basics and the page on commercial energy procurement explain how supply and delivery fit together.

How the Tag Flows Into Your Bill

Once the tag is set, it becomes a multiplier in the capacity cost calculation. The grid operator establishes a capacity value through its market processes, and your tag determines how much of that capacity obligation is assigned to your account. A larger tag means a larger share of the capacity cost; a smaller tag means a smaller share.

Because of how bills are structured, the capacity component may not appear as a clearly labeled line item. It is often folded into the supply charges or presented alongside other demand-related components. If you want to see how your peak usage affects charges more broadly, our explanation of commercial demand charges is a helpful companion, since demand charges and capacity tags both stem from peak usage but are calculated by different parties under different rules.

One point worth repeating: a grid auction or clearing price is not the same as your bill rate. The auction sets a wholesale value for capacity; your actual cost depends on that value combined with your specific tag and the terms of your supply arrangement.

Why the Tag Deserves Attention

For many commercial accounts, capacity is a real component of the total electricity cost, and it is one of the few components tied directly to your own behavior during a handful of hours each year. That makes it different from broad market prices, which you cannot control. You cannot change the grid's peak hours, but the level of load you carry during those hours is influenced by how you operate.

That is why businesses that pay attention to peak-hour operations often revisit their capacity tag each year, verify that the utility's measurement matches their own interval records, and consider whether adjustments to their operating patterns are practical. The related strategies of reducing peak-hour load are discussed in our material on peak shaving and in resources such as how storage lowers capacity and transmission costs.

Key Takeaways

• A capacity tag, or PLC, represents your facility's share of the grid's peak demand and is measured from your usage during defined peak hours in the prior year.
• The delivery utility calculates the tag within the grid operator's rules; a competitive supplier uses it but does not set it.
• The tag is generally fixed for a delivery year and resets with each new annual measurement.
• Timing of usage during peak hours, not total monthly consumption, is what shapes the tag.

Sources

• Illinois Commerce Commission
• PJM Interconnection
• ComEd
• Plug In Illinois

This article is general educational information about how capacity tags are set and does not promise any specific savings, rate, or outcome for your business.