Air conditioning: the biggest load in most homes

Typical connected load

A residential central air conditioner draws roughly 1 kW per ton of cooling capacity at the compressor, plus the indoor blower. A 3-ton system typically lands around 3 to 3.5 kW total. A 5-ton system lands around 4.5 to 5.5 kW. Older, lower-efficiency equipment draws more for the same cooling.

Many Arizona and southwestern homes have two or three separate systems. That is where the arithmetic gets ugly. Two 4-ton units starting within the same interval is 8 to 9 kW from cooling alone.

Startup inrush is much higher than running draw, sometimes five to seven times, but it lasts a fraction of a second. Because your utility averages demand across a 15, 30 or 60 minute interval, inrush does not set your peak. Sustained simultaneous operation does.

Why it spikes demand

Air conditioners are weather-driven, which means every AC in a building responds to the same conditions at the same time. On a 110-degree afternoon they are not running independently. They are all running, and they are running nearly continuously.

That is also exactly when the utility's on-peak window falls. The overlap is not coincidental. Utilities set their peak pricing windows around the hours when cooling load stresses the grid.

Thermal storage and deferrability

Air conditioning has real thermal storage, but it lives in the building rather than the equipment. The walls, floors, furniture and air in a cooled house hold what installers call cool energy. When the compressor stops, the house does not immediately warm up. It drifts.

How fast it drifts depends on insulation, outdoor temperature, sun exposure and how cool the house was when the compressor stopped. This is why the standard commissioning advice is to set the thermostat to the lowest comfortable temperature, typically 70 to 72 degrees, rather than letting the house sit at 78 and hoping. You are pre-cooling the thermal mass so there is something to coast on.

Deferrability is moderate. Minutes, not hours.

What an interruption feels like

A cycle that runs slightly shorter, or a start that happens a few minutes later than it otherwise would have. In a properly commissioned system the temperature swing is usually within a degree or two of normal thermostat behavior, because thermostats already cycle the compressor on and off constantly.

The system enforces protection limits so this stays true. On a 30-minute averaging period, an air conditioner gets a minimum ON time of 8 minutes. On a 60-minute averaging period it gets 10 minutes. Once the compressor starts, it runs long enough to actually deliver cooling to the house rather than just spinning up and shutting down. There is also a minimum OFF time of 5 minutes, which protects the compressor from short-cycling damage by letting refrigerant pressures equalize before restart.

Where it sits in the priority order

High, generally priority 2, just below the dryer. In homes with multiple systems the units are typically assigned the same priority number, which causes them to rotate rather than compete. One coasts while the other runs, then they trade. That keeps the house evenly conditioned and prevents one zone from getting warm while another stays cold.

If a customer wants one system favored, the downstairs unit for example, that one gets priority 2 and the other gets priority 3.

The honest caveat

Cooling is where you will notice demand control if you are going to notice it at all. On the worst afternoons of the year, with the demand limit set aggressively, a house can drift a degree or two above setpoint late in an on-peak window.

The fix is the demand limit dial, and it is a real trade. One documented example: a sub-3,000 square foot home with two air conditioners and a pool started at a 3.0 kW limit and saw more temperature rise than the owner wanted on 110-degree days. Raising the limit to 4.5 kW held the house at 74 degrees and still delivered roughly 40 to 60 percent bill reduction. Lower limit, more savings, less margin. That choice stays yours.

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