Demand Charges on Residential Plans and What Sets Them

Why this matters

Most residential bills charge for energy: how many kilowatt-hours the house used in the month. A demand charge bills something else: the highest rate of use the house hit, in one short interval, at any point in the billing period. Fifteen minutes of the dryer, the EV charger and the backup heat strips running together can set a charge that applies to the whole month, while the kWh total barely notices. A customer on one of these plans who gets a heat pump with electric backup, or an EV charger, can see a bill jump with no rise in kWh, and every bill-reading habit a tech has points the wrong way. The good news is that the fix is often a schedule rather than a repair.

What the demand charge measures

Demand is power, in kW, averaged over a short window called the demand interval. The tariff sets the interval, commonly 15, 30 or 60 minutes. The meter records the kWh used in every interval and converts each to an average kW: for a 15-minute interval, kWh in the interval times 4; for a 30-minute interval, times 2; for 60 minutes, kWh equals average kW.

The billed demand is usually the highest of those interval averages in the billing period. Some tariffs count every hour of every day. Others count only intervals inside an on-peak window, such as weekday late afternoons and evenings, and ignore everything else. A few use the average of several daily peaks rather than the single highest one, and some carry a ratchet that keeps billing a share of a past high month for a time.

The charge is billed demand times the demand rate, which the bill shows per kW. Write it as Rd: the demand line is the peak kW times Rd, independent of how many kWh the house used. Residential demand plans usually pair that line with a lower energy rate than the standard plan, which is how the utility makes them roughly neutral for a house whose load is spread out.

Whether a plan has a demand charge, which interval it uses, which hours count and how it is averaged are all in the tariff. Many residential customers will never see one. Before explaining a demand line, read the rate schedule named on the customer's bill (see related: Anatomy of a Residential Electric Bill, Line by Line).

What stacking looks like in one interval

         18:00 TO 18:15, ONE DEMAND INTERVAL
 +--------------------------------------------+
 | backup heat strips            10.0 kW      |
 +--------------------------------------------+
 | EV charger                     7.2 kW      |
 +--------------------------------------------+
 | clothes dryer                  5.0 kW      |
 +--------------------------------------------+
 | heat pump compressor           3.0 kW      |
 +--------------------------------------------+
 | base load (fridge, lights)     0.8 kW      |
 +--------------------------------------------+
  all five running the full 15 minutes
  average for the interval = 26.0 kW
  energy in the interval   = 26.0 x 0.25 = 6.5 kWh

The load values are illustrative; real ones come from nameplates and, better, from measurement (see related: How to Measure Real Watts Instead of Trusting the Nameplate). The point of the picture is that nothing in it is unusual. Each of those loads is normal on a cold weekday evening. Together they draw 26.0 kW for a quarter of an hour, and if that is the highest interval of the month, the demand line bills 26.0 times Rd.

Look at the energy: 6.5 kWh. In a house using well over a thousand kWh a month in winter, that interval is a small fraction of the energy bill and the whole of the demand bill.

Worked example: same kWh, staged loads

Here is the house above on a demand plan with a 15-minute interval that counts every hour of the day. The household changes nothing about what it uses, only when.

Before. Billed demand is set by the evening interval: 26.0 kW. Demand line: 26.0 Rd.

After staging.

• The EV charger is scheduled to start at 23:00, when the heating load is lower and nothing else big is on. That interval holds the EV at 7.2 kW, the heat pump at 3.0 and the base load at 0.8: 11.0 kW.
• The dryer runs in the early afternoon, with the heat pump and the base load: 5.0 plus 3.0 plus 0.8 is 8.8 kW.
• The backup strips still come on during the morning warm-up and a defrost cycle, with the heat pump and the base load: 10.0 plus 3.0 plus 0.8 is 13.8 kW.

The highest interval of the day is now 13.8 kW, the morning warm-up. Demand line: 13.8 Rd.

The change. 13.8 divided by 26.0 is 0.53. Billed demand fell 47 percent, from 26.0 to 13.8 kW. Check the difference the other way: 26.0 minus 13.8 is 12.2 kW, which is the EV charger and the dryer together (7.2 plus 5.0). Those were the two loads moved out of the strips' interval, so the subtraction confirms the staging, not a coincidence.

What did not change. The kWh. Every load still ran for the same time. The energy lines on the bill are the same; the demand line is about half.

The limit. The morning strip interval is now the peak, so the next target is the backup heat. That is an HVAC question rather than a scheduling one: how often the strips run, whether they stage, and what outdoor temperature they are locked out above. Changing that lockout trades bill for comfort and, in very cold weather, for the house's ability to hold temperature at all, so it is set by the HVAC tech against the heat pump's capacity at that temperature, not by the office. Where the strips are running far more than the weather explains, the fault is a separate investigation (see related: How to Confirm Backup Heat Runtime From the Thermostat and a Clamp).

What changes the answer

An on-peak-only demand window. If only, say, weekday evening intervals count, then moving the EV charger to 23:00 removes it from the demand calculation entirely, not just from the stack. Staging matters only inside the window. A load that runs outside the window, at any size, costs energy only.

A longer interval. On a 60-minute interval, a dryer that runs 20 minutes contributes a third of its kW to that hour's average, not all of it. Short bursts matter less; long overlaps matter more. On a 15-minute interval, almost every overlap counts at full weight.

Averaging several peaks. A tariff that bills the average of the highest interval on each of several days punishes a habit (the same stack every evening) more than a single bad day. A one-off overlap, like a holiday with the oven and the dryer and guests, hurts less than it would on a single-peak plan.

A ratchet. Where one applies, a single high month can keep raising bills for months after. Ratchets are far more common on commercial tariffs than residential ones; check before mentioning one to a homeowner.

A load management device. Some EV chargers and load centers can throttle or pause a load while another big one runs. This is a design and installation question for an electrician, including whether the device is listed for the purpose and how it interacts with service sizing. Do not suggest bypassing or defeating one to fix a comfort complaint.

Reading the demand line on the bill and in the data

On the bill. Look for a line in kW, not kWh, often with the date and time the peak occurred. That timestamp is the single most useful fact on the bill, because it tells you what the household was doing.

In interval data. If the customer's usage portal offers 15-minute data, multiply each interval's kWh by 4 to get average kW, and find the highest. Hourly data cannot show a 15-minute peak: an hour containing one 26.0 kW quarter and three 8.0 kW quarters averages 12.5 kW, which hides the interval the demand line billed (see related: Smart Meter Interval Data: What the Hourly Record Shows). Check: 26.0 plus 8.0 three times is 50.0; divided by four is 12.5.

On the meter. Some meters display the month's maximum demand on one of their screens, which resets each billing period.

Where readers go wrong

Looking only at kWh. A demand-plan bill can rise with flat kWh. If the energy lines held and the bill moved, the demand line is the first place to look.

Blaming the biggest single load. The 10 kW strips alone did not set the 26.0 kW peak; the stack did. Moving the two smaller loads out of that interval did more than any repair to the strips could.

Telling a customer to "use less." On a demand plan, the advice is "use it at a different time." The kWh may not need to change at all.

Assuming the customer is on a demand plan because a sibling customer was. Demand plans are optional in some territories, default for some new customers in others, and absent in most. Confirm from the bill.

Promising a figure. Staging cut billed demand here by 47 percent on illustrative loads. The real result depends on which loads overlap, the interval, the window, and what the household is willing to change. Say what the change does to the peak in kW and let the bill confirm it (see related: Savings Claims Without Overpromising: The Honest Language).

How to verify the analysis

• The bill shows a demand line in kW and a rate per kW, and you know the interval and the hours that count.
• The kW you compute for an interval is its kWh times 60 divided by the interval length in minutes.
• The peak you identify has a time you can match to something the household does.
• After a staging change, the next bill's demand line falls by at most the kW you moved out of the peak interval (it lands on whatever interval is now highest), with kWh roughly unchanged.

References

• See related: kW Versus kWh: The Difference That Confuses Every Bill Call
• See related: Smart Meter Interval Data: What the Hourly Record Shows
• See related: Time-of-Use Rates and Equipment Settings
• See related: How to Confirm Backup Heat Runtime From the Thermostat and a Clamp
• The utility's filed residential demand rate schedule, which sets the demand interval, the hours that count, the averaging method and any ratchet