If your ComEd account covers more than one meter, the demand charge is computed in a way the bill does not explain, and the natural reading of the bill is wrong in a direction that costs you effort in the wrong place.
The bill prints a demand value next to each meter. It is extremely tempting to add them up and treat the total as your peak. Do not. Those per-meter numbers are not each meter's maximum.
What the numbers on the bill actually are
Billed demand on a multi-meter ComEd site is site coincident. ComEd looks across all the meters at the site, half hour by half hour, adds them together at each interval, and takes the highest of those sums inside the on-peak window. That sum is your billed demand.
The per-meter figures printed on the bill are each meter's reading at the moment of that site-wide peak. They are a decomposition of one instant, not a set of independent maxima.
That distinction is not cosmetic. It is the difference between two questions:
- When was each piece of my site at its worst? Interesting. Not billed.
- When was my whole site at its worst, all at once? Billed.
We confirmed this by reconstructing a two-meter ComEd site's interval data against its bill. The site-coincident half hour came to 40.91 kW, the bill's demand line matched it exactly, and the Distribution Facilities Charge reconciled to $596.88. Each meter's own highest reading of the period was higher than the number printed against it, because each meter peaked at a different moment than the site did.
Why this is good news, and what it makes possible
Coincidence is a discount you are already receiving. If your two meters peak at different times, you are billed on the sum at one instant, which is lower than the sum of the two maxima. Every hour of separation between your loads is already money you are not paying.
It also names a lever that only exists on multi-meter sites: staggering across meters. If the load on meter A can be moved so that it does not overlap the load on meter B, the coincident sum falls even though nothing was turned off and no energy was saved. That is unusual. Most demand work asks you to do less; this asks you to do the same things at different times.
And it caps itself in the same way everything else does. Separating your two largest loads lowers the coincident peak only until some other combination of equipment becomes the highest coincident half hour. The saving is the gap down to that next combination, not the size of what you moved.
Where the misreading costs real money
Two failure modes, both common, both avoidable from the bill you have.
Sizing equipment against an inflated peak. Add the per-meter numbers, get a figure well above your actual billed demand, and every piece of equipment quoted against it is oversized. On a battery that is the difference between a project that pencils and one that does not, because the capital scales with the number and the saving does not.
Attacking the wrong meter. The meter with the largest printed number is not necessarily the one contributing most to the coincident peak. A meter that runs hard at seven in the morning may contribute almost nothing at the instant that sets the bill. Reducing it feels productive and changes nothing.
Both errors have the same root: reasoning about the site's peak from numbers that describe one instant.
What you can establish today
Check whether your account is multi-meter. The bill will list them. If it lists one, none of this applies and your billed demand is simply that meter's highest qualifying half hour.
Compare the printed per-meter values against your billed demand. If the billed demand equals the sum of the printed values, you are looking at the coincident decomposition, exactly as described.
Ask whether your two largest loads are on different meters, and whether they run at the same time. You know the answer without any data. If they overlap and either is movable, there is a lever here. If they are already separated, the coincidence discount is already yours and there is nothing further to win from timing.
Turning that into a number means sorting the summed intervals from the interval data behind the bill and finding what the coincident peak would become after a change. That is a measurement of a site as it actually ran, not a projection, and it is the only version worth spending capital against.
How does ComEd calculate demand for a site with multiple meters?
By summing the meters at the same half-hour interval and taking the highest of those sums during the on-peak window. It is a coincident measurement across the site rather than a sum of individual maxima.
Are the per-meter demand numbers on my bill each meter's highest reading?
No. They are each meter's reading at the moment the site as a whole reached its coincident peak. Each meter's own maximum is usually higher and usually occurred at a different time.
Does moving load between meters reduce my demand charge?
Moving load from one meter to another changes nothing by itself, because the billing sums them. Moving load to a different time, so that it no longer coincides with the other meters' load, does reduce the coincident peak.
Should I ask ComEd to separate my meters onto different accounts?
That is a different question with billing, tariff and metering consequences beyond the demand charge, and it is not answerable from the demand mechanism alone. Establish first whether coincidence is actually costing you anything, which requires knowing what your peak would be under each arrangement.
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