Hot summer days can mean high electricity bills. People want to stay comfortable without wasting energy and money. Maybe your household has fought over the best strategy for cooling your space. Which is more efficient: running the air conditioning all summer long without break, or turning it off during the day when you’re not there to enjoy it?
We are a team of architecturaland building systemsengineers who used energy models that simulate heat transfer and A/C system performance to tackle this perennial question: Will you need to remove more heat from your home by continuously removing heat throughout the day or removing excess heat only at the end of the day?
The answer boils down to how energy intensive it is to remove heat from your home. It’s influenced by many factors such as how well your house is insulated, the size and type of your air conditioner and outdoor temperature and humidity.
According to our unpublished calculations, letting your home heat up while you’re out at work and cooling it when you get home can use less energy than keeping it consistently cool – but it depends.
First, think about how heat accumulates in the first place. It flows into your home when the building has less stored heat than outside. If the amount of heat flowing into your home is given by a rate of “1 unit per hour,” your A/C will always have 1 unit of heat to remove every hour. If you turn off your A/C and let the heat accumulate, you could have up to eight hours’ worth of heat at the end of the day.
It’s often less than that, though – homes have a limit to how much heat they can store. And the amount of heat that enters your home depends on how hot the building was to begin with. For example, if your home can only store 5 units of thermal energy before coming to an equilibrium with the outdoor air temperature, then at the end of the day you will only ever have to remove 5 units of heat at most.
Additionally, as your home heats up, the process of heat transfer slows down; eventually it reaches zero heat transfer at equilibrium, when the temperature inside is the same as the temperature outside. Your A/C also cools less effectively in extreme heat, so keeping it off during the hottest parts of the day can increase overall efficiency of the system. These effects mean there’s no one straightforward answer to whether you should blast the A/C all day or wait until you get back home in the evening.
Energy used by different A/C strategies
Consider a test case of a small home with typical insulation in two warm climates: dry (Arizona) and humid (Georgia). Using energy modeling software created by the U.S. National Renewable Energy Laboratory for analyzing energy use in residential buildings, we looked at multiple test cases for energy use in this hypothetical 1,200 square-foot (110 square-meter) home.
We considered three temperature strategy scenarios. One has the indoor temperature set to a constant 76 degrees Fahrenheit (24.4 degrees Celsius). A second lets the temperature float up to 89 F (31.6 C) during an eight-hour workday – a “setback.” The last uses a temperature setback to 89 F (31.6 C) for a short four-hour workday.
Within these three scenarios, we looked at three different A/C technologies: a single stage central A/C, a central air source heat pump (ASHP) and minisplit heat pump units. Central A/C units are typical of current residential buildings, while heat pumps are gaining popularity due to their improved efficiency. Central ASHPs are easily used in one-to-one replacements of central A/C units; minisplits are more efficient than central A/C but costly to set up.
We wanted to see how energy use from A/C varied across these cases. We knew that regardless of the HVAC technology used, the A/C system would surge when the thermostat setpoint returned to 76 F (24.4 C) and also for all three cases in the late afternoon when outdoor air temperatures are usually the highest. In the setback cases, we programmed the A/C to start cooling the space before the resident is back, ensuring thermal comfort by the time they get home.
What we found was that even when the A/C temporarily spikes to recover from the higher indoor temperatures, the overall energy consumption in the setback cases is still less than when maintaining a constant temperature throughout the day. On an annual scale with a conventional central A/C, this could result in energy savings of up to 11%.
However, the energy savings may decrease if the home is better insulated, the A/C is more efficient or the climate has less dramatic temperature swings.
The central air source heat pump and minisplit heat pump are more efficient overall but yield less savings from temperature setbacks. An eight-hour setback on weekdays provides savings regardless of the system type, while the benefits gleaned from a four-hour setback are less straightforward.
“By turning it off, you're making your A/C go full-throttle every single time it's turned on, and that gobbles up much more electricity than just maintaining the desired temperature.” For most households, a strategy called “temperature setbacks” is the most effective way to cut costs, pros say.
Our HVAC technicians recommend turning up your thermostat by 7-10 degrees while you're out of the house, instead of turning it off entirely! Below, our HVAC Company describes the damage to your home turning off your AC and the resulting high humidity can cause.
The short answer is yes, it is cheaper to leave your air conditioner on all day. But this does not mean that you should leave it on full blast, keeping your home so cold that you need a sweater or jacket to be comfortable.
Although many people believe that it is more costly to turn your AC on and off vs. leaving it on, this is entirely false. People often base this belief on the idea that powering on your system requires a burst of energy, so leaving it running would minimize energy usage by reducing these bursts.
When you notice that your air conditioner isn't cooling, your first action should be to turn it off. This might seem counterintuitive—after all, you want your home to be cool. However, continuing to run an AC that isn't effectively cooling can cause further damage to the system.
In addition to cooling down the home, ACs also remove humidity. If your air conditioner is not running for hours, your home can become extremely humid. This is bad for both home furnishings and the house itself. Mold and mildew are more likely to grow, and wood or paper can warp.
Tips For HVAC Thermostat Temperature Settings on Vacation
Typically, you should set your temperature no higher than 84 degrees when you're on summer vacation and the outdoor temperature is in the high 90s. Spring and fall vacations can vary.
For many of us, 70 degrees is the ideal indoor temperature, but when it is close to 100 degrees outside, your AC unit will be working quite hard to meet these expectations. Consider setting your thermostat at 75 degrees to 80 degrees in your home if the outside temperature is approaching triple digits.
To begin, let's clarify one of the most common misconceptions about AC efficiency. Contrary to popular belief, turning your AC on and off frequently does not consume more energy than leaving it on continuously. In fact, the opposite is often true.
The U.S. Environmental Protection Agency suggests that people set their air conditioners to 78 degrees in summer while they are home for maximum savings on their utility bills. However, comfort is subjective, and what works for one household may not work for others.
This means, that in turn, when your setting is turned on auto less electricity is used. When your fan and air conditioner are running all the time, more electricity is produced. All in all, it is better to leave your air conditioner's fan on auto, instead of “On” at all times.
Turning your air conditioner in and off forces it to run at lower speeds for shorter periods, which ends up worse for you because now you have both a hot house and a high energy bill. It also adds extra strain on your unit, which can age it prematurely, resulting in you needing an emergency AC replacement.
The age-old question of whether to turn off your air conditioning when you're not at home is one that many homeowners grapple with. Contrary to popular belief, leaving your AC on while you're away can actually save you money and protect your home from various issues.
When you use the remote to turn off the AC, the unit goes into standby mode. In this mode, the AC stops cooling but remains on standby to respond to the remote when you want to turn it back on quickly. However, even in standby mode, the AC continues to consume a small amount of electricity.
AUTO is significantly more energy efficient between the two modes since the fan is only running when your AC is going through its cooling cycle. This takes less power, which is more budget-friendly, with some estimates showing you can save as much as $50 per month just by keeping your AC on AUTO.
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Introduction: My name is Reed Wilderman, I am a faithful, bright, lucky, adventurous, lively, rich, vast person who loves writing and wants to share my knowledge and understanding with you.
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