Most households are billed for how much electricity they used. A growing number are also billed for how fast they used it at one particular moment, and that second charge can be the larger of the two in a hot month.
How a Demand Charge Works
Energy and demand are different quantities. A kilowatt hour measures a quantity of electricity. A kilowatt measures the rate at which it is being drawn. Commercial customers have been billed on both for decades. What is newer is residential rate plans that do the same.
The mechanism is simple and slightly brutal. The meter records your average draw over a fixed interval, often thirty minutes, and keeps the highest one from the billing period. That single interval sets a charge for the month, regardless of how careful you were the rest of the time.
The figures are not trivial. On one widely used plan in the southwest, summer demand is priced in tiers, with the first few kilowatts costing under ten dollars each and the higher tiers running toward thirty dollars per kilowatt during the hottest months. A household peaking at eight kilowatts in a July afternoon can see well over a hundred dollars in demand charges alone before a single kilowatt hour is counted.
What creates a peak is almost always coincidence rather than consumption. The air conditioner is running, the oven is on, someone starts the dryer, and the EV begins charging, all within the same half hour. Each one is ordinary. Together they set a number that is billed for a month.
Most plans only measure demand during defined on-peak windows, typically weekday afternoons in summer and both ends of the day in winter. Outside those hours the same simultaneous load costs nothing extra, which is what makes the charge manageable rather than merely expensive.
This is also the rate design under which a home battery stops being a resilience purchase and becomes an arithmetic one. A battery that discharges during the peak window flattens the measured draw regardless of what the household is doing, and on a demand plan that is worth real money every month rather than only during an outage.
The Hardware Stack:
Emporia Vue: A panel monitor that reports demand in the same interval the utility uses, which is the only way to see your own peak as it forms.
SPAN Smart Panel: Circuit-level control, which can delay or shed a large load automatically rather than relying on anyone noticing.
Tesla Powerwall 3: A battery with enough continuous output to cover a household's on-peak draw, which is what converts a demand charge into a storage decision.
Vulnerability Score
One bad half hour: A single coincident interval sets the month's charge, so careful behaviour on every other day does not reduce it.
Invisible formation: Without interval monitoring a household cannot see its own peak, and the bill arrives weeks after the moment that caused it.
Electrification pressure: Adding an EV charger or a heat pump to a demand-billed account raises the peak much more than it raises consumption, which surprises people who budgeted on kilowatt hours.
Finding Out Whether This Applies to You
Look at your bill for a line item quoted in kilowatts rather than kilowatt hours, or for the word demand. Many utilities also publish which of their residential plans include one, and some have moved new customers onto such a plan by default.
If you are on one, the cheapest response is sequencing rather than equipment. Run the dryer in the evening, start the EV charge after the window closes, and avoid stacking the oven onto an afternoon when the air conditioner is already working. If you are considering a battery anyway, ask the installer to model it against your demand charges specifically, because that calculation looks quite different from the usual one built on energy rates.
Written by Mason Vance
