Going green without going broke

Electrification works. A heat pump moves heat instead of making it, so it delivers more heating per dollar than a furnace in most climates. An EV costs a fraction of a gasoline car per mile. An induction range is faster and cleaner than gas. Rooftop solar cuts the number of kilowatt-hours you buy from the utility, sometimes to near zero on a good day.

None of that is in dispute. What surprises people is the bill.

A homeowner adds solar. Then a Level 2 charger. Then a heat pump. Then an induction range. Every one of those decisions is defensible on its own. Twelve months later the total bill has not moved the way the spreadsheet said it would, and in some cases it has gone up. The homeowner did everything right and got punished for it.

The reason is that most people are optimizing one number on their bill while a second number quietly runs the show.

Two numbers on one bill

Utility bills for customers on a demand rate have two distinct charges built from two distinct measurements.

The first is energy, measured in kilowatt-hours (kWh). This is the total amount of electricity you consumed over the month. It is a volume measurement, like gallons of water.

The second is demand, measured in kilowatts (kW). This is the highest rate at which you pulled electricity at any single moment during the month, averaged over a short interval — 30 minutes on SRP, 60 on APS, 15 on some commercial tariffs. It is a flow measurement, like the width of the pipe.

The utility bills you for both because it has to build for both. Energy is what it has to generate. Demand is what it has to be ready to deliver at your highest demand interval, every day of the year, whether you use it or not.

Here is the part that catches people. Solar and batteries are kWh products. Demand control is the kW product. They address different lines on the bill.

Solar is genuinely good at what solar does

This is not a knock on solar. Solar does its job. It reduces the kilowatt-hours you buy, it hedges against rate escalation, and on many rate structures it pays back. If you have solar, keep it.

But solar's output is a function of the sun, and the sun is not consulted about when your appliances turn on. Cloud cover, monsoon season, panel soiling, snow, and inverter faults all produce production gaps. Those gaps do not politely avoid your peak hours. In Arizona a summer afternoon storm can drop array output sharply at exactly the hour the air conditioner is working hardest. If that moment lines up with the dryer and the EV charger, the meter records a demand peak, and that peak sets the demand charge for the entire billing month.

Batteries help, and a large enough battery with the right control logic can shave demand. But the battery has to be sized, charged, and dispatched for that purpose, and it has to still have charge left at the moment the peak arrives. That is a different design problem than backup power or evening self-consumption, which is what most residential batteries are configured for. See our guide on why solar alone does not fix a demand charge for the full picture.

Every new appliance is another candidate for the same demand interval

The arithmetic of demand billing is unforgiving. It does not care how many hours a day your appliances are off. It cares about one interval.

Consider what electrification adds to a house:

• A 48-amp Level 2 EV charger draws roughly 11.5 kW while charging. A 32-amp unit draws roughly 7.7 kW.
• An air-source heat pump with electric backup heat can pull well into the double digits in kW when the strip heat stages come on during a cold snap or a defrost cycle.
• An induction range with multiple elements and the oven running is a meaningful block of load on its own.
• The water heater, the dryer, and the pool pump were already there.

Before electrification, a home might have had two or three loads large enough to move the demand needle. After electrification it has six or seven. The number of ways those loads can overlap inside a single demand interval rises faster than the number of loads does. Nobody schedules the collision. The car plugs in on arrival, the heat pump runs on thermostat, the water heater runs on its own thermostat, and the range runs on dinner. They are independent systems that happen to share a meter.

Our guides by load type walk through how each major appliance behaves under demand control.

What demand coordination actually does

The Energy Sentry DAC watches the whole service, not individual circuits. Current transformers on the incoming mains, at roughly a 1,000:1 ratio, feed the controller a live picture of everything the building is pulling. It samples that 1,000 times per second.

The important part is what it does with those samples. It does not simply compare demand against a limit and react after the limit is crossed. It tracks the rate at which demand is climbing, projects where the interval is heading, and acts before the limit is reached. When it sees a peak forming, it briefly holds back the lowest-priority loads.

Loads with thermal storage are ideal for this. A water heater tank holds hot water whether the element is energized or not. A house holds temperature for minutes after the compressor pauses. Deferring those loads for a few minutes is invisible to the people living there.

Be clear on what that means: the DAC defers load, it does not eliminate it. The water heater still reheats. The car still charges. The kilowatt-hours are still consumed and still billed. What changes is the shape of the curve, which is the only thing the demand charge measures.

It is also not a smart breaker, and not a monitoring app that shows you a chart and asks you to change your behavior. It runs automatically and continuously without anyone in the house doing anything.

The honest version of the pitch

Electrification is the right direction. It is also a load-coordination problem that most homes were never wired to solve, and the demand determinant on the bill is where that shows up.

Solar handles the kilowatt-hours. Demand control handles the kilowatts. On a demand rate, you need both to get the result the brochure promised.

Energy Sentry has been building demand controllers since 1978, with more than 35,000 installations in the field. The 9388A Demand Automation Computer carries a 3-year warranty.

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Questions about your own setup? Call toll free (888) 461-9336, direct (970) 461-9600, or email help@energysentry.com. Monday–Friday, 8:00 AM–5:00 PM Arizona time.