Does it really save electricity to turn the air conditioner off when you leave the house?

Do you need to turn the air conditioning on full blast even when you're not there?

Intense heat throughout the summer months might result in hefty electrical costs. Individuals seek cost-effective methods of maintaining their comfort levels. Perhaps you and your loved ones have argued over which cooling method is more effective. Is it more cost-effective to leave the air conditioner on all day during the summer, or switch it off while you're not around?

Architectural and building systems engineers here to answer the age-old question: is it better to remove heat from the house constantly throughout the day or to remove surplus heat just at the end of the day using energy models that simulate heat transfer and A/C system performance?

The amount of energy required to remove heat from a home is the determining factor. It depends on a number of things, including the quality of your home's insulation, the size and type of air conditioner you have, and the ambient temperature and humidity outside.


Let your house warm up while you're at work and turn the air conditioning down before you arrive home; our internal research shows that this may be more efficient than maintaining a constant chilly temperature.


Turn on the air conditioning and leave it on all day?

You need to first consider how heat builds up. When the temperature within your house is lower than the outside temperature, heat will flow into the house. Assuming a constant rate of "1 unit per hour" for the inflow of heat into your home, your air conditioner will always have 1 unit of heat to remove every hour. Turning off your air conditioner and letting the heat build up can provide you with enough warmth for up to eight hours after the sun goes down.

The heat storage capacity of most houses, however, is substantially lower. How much heat is transferred into your home is also influenced by how hot the structure was to start with. For instance, if the maximum amount of heat your house can store before reaching equilibrium with the ambient air temperature is 5, then the maximum amount of heat you would ever have to remove from your house each day is 5.


The rate of heat transfer also decreases when the temperature inside your home rises, and it stops entirely once the interior and exterior temperatures have equalized. In addition, your air conditioner is less effective at its job when the temperature outside is very high, so turning it off during the hottest portions of the day can improve its effectiveness. Because of these factors, there's no simple solution for deciding when to turn on the air conditioner during the day and when to wait until nightfall.


Energy consumption of various air conditioning methods

Let's use a tiny house with standard insulation in two different kinds of hot climates as a test case: one dry (Arizona) and one humid (Georgia). We examined a number of scenarios for the energy consumption of this made-up 1,200 square foot (110 square meter) home using energy modeling software developed by the United States National Renewable Energy Laboratory to analyze residential building energy consumption.


There were 3 possible temperature strategies we thought about. For one, he or she maintains a steady temperature of around 75 degrees Fahrenheit within the house (24.4 degrees Celsius). It's a "setback" if the temperature rises to 89 degrees Fahrenheit (31.6 degrees Celsius) during an eight-hour workday, as it does in the second scenario. The final option involves reducing the temperature to 89 degrees Fahrenheit (31.6 degrees Celsius) and working for only four hours per day.


We analyzed three distinct types of air conditioning systems across these three cases, including single-stage central air conditioning, air source heat pumps (ASHPs), and mini-split heat pumps. Modern homes typically feature central air conditioning, although heat pumps are on the rise as a result of their increased efficiency. Minisplits are more efficient than central air conditioning but more expensive to install, while central ASHPs can be used to replace individual central air conditioning units.


We were interested in comparing the energy consumption of air conditioners in these different scenarios. In all three scenarios, the hottest time of day was in the late afternoon, so we anticipated an A/C surge when the thermostat was reset to 76 degrees Fahrenheit (24.4 degrees Celsius). We made sure the resident will be comfortable at their home's temperature by setting the air conditioner to kick on before they returned from their setback.

If you want to know how much energy your Phoenix, Arizona home will need during the hot, dry summer months, you can use an energy model to find out. The study compared the costs of three distinct HVAC systems and six distinct methods of regulating indoor temperature.


This time, for a residence in hot and humid Atlanta, the researchers used the same three different HVAC technology and three different temperature-setting procedures.

We discovered that in the setback scenarios, energy consumption is still lower than when maintaining a constant temperature throughout the day, even when the A/C suddenly surges to recover from the higher inside temperatures. Energy savings of up to 11% per year are possible when compared to the use of a traditional central air conditioner.


If the house is well-insulated, the air conditioner is efficient, or the local climate has few extremes in temperature, the energy savings may be reduced.


Mini-split heat pumps and central air source heat pumps are more energy efficient overall, but they don't save as much as conventional HVAC systems when you turn the heat down. On weekdays, an eight-hour setback results in cost reductions for any system, whereas the four-hour setback's advantages are more subtle.


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