What Is Home Precooling, and for Whom Is It Useful?

Desperate times call for desperate measures, you may have heard. That may sound a bit melodramatic, but after the hottest summer on record, many of us have just paid out the most we’ve ever paid on our utility bills. Air conditioning uses a lot of electricity, and even with the temperature setting turned up higher than we’d like, we may still have to endure warm indoor temperatures just so the bill isn’t any higher.

But what if there were a fairly simple tactic that could lower the electric bill and keep your home’s temperature within a comfortable range? That tactic is called precooling, and here’s how it works.

Cooling During Off-Peak Hours

The primary advantage of precooling comes with time-of-use (TOU) electricity pricing. TOU is aimed at load reduction—getting people to use less electricity during peak demand hours. TOU pricing just means that you pay a peak rate at times of highest demand, which is usually the hours after the typical workday, around 4–7 PM. People get home from work, turn up the AC, start making dinner, and so on. Electricity demand spikes and many utilities are forced to fire up their natural gas peaker plants. Some utilities also initiate their demand-management options like OhmConnect, as well as EnergyHub, which operates in Arizona and works with Tucson Electric Power. More on that in a moment.

If you were to practice precooling at home, you would cool your house to a lower-than-normal temperature prior to the times of peak rates. Then you would get home from work and find that you don’t need to turn up the AC. Precooling probably works best in hot-dry climates like the Southwest U.S. experiences, where there’s usually low humidity and nighttime temperatures are substantially lower than daytime high temperatures. Installing a smart thermostat will make this practice as automatic as possible, as the thermostat will learn when you’re precooling and start doing it for you. For example, you may not do it on the weekends.  

Besides demand management, this practice also helps utilities make use of ultra-cheap solar power that’s available earlier in the day that would otherwise not be needed.

Tucson Electric Power is one of the utility companies that advises its customers on how to practice precooling. Here’s what they say:

• Summer on-peak hours are 3–7 p.m. on weekdays from May through September. For precooling, set your thermostat 3° lower than your preferred temperature from noon to 3 p.m.
• When on-peak hours begin, adjust the thermostat to a few degrees higher than your preferred temperature and leave it there until 7 p.m. when on-peak hours end.
• Their example is as follows: your preferred temperature is 78°. You precool the house to 75° from noon to 3 p.m. and then raise the temperature to 81°. You leave the thermostat there until 7 p.m. and then you can drop the temperature to a more comfortable setting.
• Using a smart thermostat will make all of this easy and automatic.

Does Precooling Work for Every Home?

Unfortunately, precooling doesn’t work as well for highly inefficient homes. If your home has a lot of air leakage, your cool, dehumidified air will leak out during the day and the temperature will gradually increase. You could still come out ahead, though. If the temperature when you arrive home is less with precooling than without precooling, and you save more money by using off-peak electricity, it doesn’t have to work out perfectly to be worthwhile. You might just have to experiment with different times and temperature ranges for precooling while you monitor your electricity usage daily. You should be able to check that at your provider’s website. You might need to precool for another hour, or take it down two more degrees, for example, to be comfortable when you get home.

Plus, you can make meaningful improvements to air sealing in just a few hours with caulking and weatherstripping. Improving your home’s insulation is usually a bigger project, but it is definitely worth the cost and effort.

Other tips for maximizing the effectiveness of precooling include installing a smart thermostat, which will learn your precooling program and manage it for you. You should also get into the habit of closing blinds on the south and west sides of the house to keep the heat from the blazing sun out.

What About Supercooling?

Some people are taking precooling a step further with what they call “supercooling.” This is simply the practice of taking the temperature waaaaay down at night or early in the day. No doubt you’ll need to try this for yourself to see where you find the sweet spot of comfort and energy savings.

Here’s the actual practice of one homeowner in Arizona:

• At 7 p.m. the thermostat drops the temperature to 75°.
• At 6 a.m. the thermostat drops the temperature to 70°.
• At 11:55 a.m. the thermostat raises the temperature to 85°. He said the highest indoor temperature has been 81°, even with outside temperatures of 110°.
• The family never runs any major appliances during peak hours. They run the laundry at night or in the morning, for example.

So that’s six hours with the thermostat set at 70°. According to the homeowner, he was driven to try supercooling after a $340 July electric bill. After using supercooling this way, he saved about $80 on his next bill.

Other Options to Consider

You can also look into battery storage systems. Installing a system like this enables you to optimize your usage of grid electricity and use only the cheapest electricity. Here’s how that would work. At night, you would fully charge your battery storage system with off-peak grid electricity. Early in the day, you would precool your home with the cheapest grid electricity on offer. Your smart thermostat and app for your battery system can manage that for you. Then, when your provider switches over to on-peak rates, your smart thermostat or app turns your AC off and monitors your home. You have the option to use your own battery power to run your AC, which you might want to do during historic and dangerous heat waves.

You also have the option to sell your battery power back to your provider at a premium rate and recoup some of your investment in your battery system. If you have a well-insulated home and have adequately precooled it, you might find the temperature perfectly comfortable through peak-rate hours. You’re buying low and selling high. That sounds like a great option to have!

Precooling Supports Comfort and Health

After record heat this past summer across most of the U.S., it’s important to consider all of our options. Excessive heat is flat out dangerous to many people with health conditions. It’s also a huge financial strain for millions of people, and many of our electric providers have had trouble meeting demand. If precooling can make a difference for these challenges, it’s certainly worth a try.