If you live in an old house or apartment without air conditioning, hot summer days can be brutal. And when air quality is bad (like during wildfire season), opening the windows isn’t a great idea. About two-thirds of households in the United States have central air conditioning, and another quarter of households rely on window-mounted air conditioners for cooling. (Those numbers are expected to rise as folks upgrade central HVAC systems to heat pumps). Fortunately, there are good options for temporary window-mounted air conditioners available for nearly anyone and any window. When you’re shopping for a new window air conditioner, the first thing you should do is figure out what size window air conditioner you need.
Generally, window air conditioners are sized based on BTU. BTU stands for British Thermal Unit, and it’s a measure of how much energy is needed to raise the temperature of one pound of water one degree Fahrenheit at zero feet (sea level) elevation. What you need to remember is that a BTU rating for an air conditioner is that the larger your indoor space, the more BTUs you need.
A word of warning: More isn’t always better. Significantly oversizing a traditional window air conditioner for the space it is cooling can cause the air conditioner to operate less efficiently than a properly-sized unit. The compressor in the air conditioner will cycle on and off more frequently (because it will quickly cool the room), and that frequent on/off cycle not only leads to higher electricity consumption, but also premature failure of the compressor. Inverter air conditioners handle oversizing better (more on that later), but it’s always a good idea to properly size your air conditioner, for both budget and performance.
The rule of thumb is that you need about 20 BTUs per square foot. Keep in mind that this metric makes some assumptions about ceiling height in your home, and it’s more about ballparking than an exact number. Rooms that get a lot of sunlight or are on upper stories tend to be hotter, and it’s generally a good idea to size up your air conditioner BTU needs by about 10% to accommodate. If you really want to know how much air conditioning (and heat) your home needs, you can even run your own Manual J load calculation.
In the United States, Energy Star, administered by the EPA (Environmental Protection Agency), partners with thousands of organizations to rate the energy efficiency of all kinds of products, including window air conditioners. This includes a label on the packaging that shows an air conditioner’s estimated annual energy use expressed in kilowatt hours per year (kWh/yr) and its Combined Energy Efficiency Ratio (CEER). Keep in mind, the estimated energy usage is for normal, intermittent use (and not if the AC was running 24 hours a day all year).
Annual energy usage isn’t the sole metric you should look at for efficiency. A better metric to pay attention to is the CEER number. This metric takes into account the amount of cooling alongside energy used. The higher the CEER number, the more efficient the air conditioner is. In order for an air conditioner to qualify for an Energy Star rating, it needs to have a CEER rating of 12 or higher. The most energy efficient window units on the market today have CEER ratings of 15 or more.
Up until recently, window air conditioners had pretty basic functionality, with the compressor able to just be on or off. That turning on or off is triggered based on the temperature in the room. That rudimentary technology does cool the space, but a new technology has made its way into window air conditioners: inverters. Rather than cycling on and off, an inverter air conditioner lets the compressor run continuously, ramping up and down instead of turning on and off (another term for this is variable speed). It’s a more energy efficient approach, letting the air conditioner react more precisely to changes in temperature. Where an on/off air conditioner might need to see a 5 degree rise in temperature before turning back on, an inverter air conditioner might only need to see a one-degree rise in temperature before turning back on, meaning it needs to cool less, and is better at maintaining a consistent temperature.
One other benefit of inverter air conditioners, is that they do a better job of dehumidifying air than their on/off counterparts (due to the fact that the compressor is running continuously). They’re also much quieter than their on/off counterparts. Of course, this is relatively new technology and an air conditioner sized for the same cooling load as an on/off air conditioner is more expensive. However, they’re up to 30% more efficient or more than traditional window air conditioners and tend to last longer and require less maintenance. That initial up-front cost turns into significant savings over the life of the air conditioner.
Energy Star has a great tool to sort window air conditioners by efficiency. However, being able to find an air conditioner when you need it is another matter. The best time to purchase an air conditioner is before the cooling season starts and stores have units in stock. Here are five models that caught our attention, both for their energy efficiency, as well as handy features such as remotes and mounting options that use less of the window. Many offer different sizes (cooling capacity) in their model ranges, too, often with the same or similar energy efficiency. They are also all inverter air conditioners, a trait the most efficient window air conditioners have in common.
You may notice visual similarities between units with very similar ratings, and it’s likely that window air conditioners in this size and price range are manufactured by the same company and white-labeled for the seller. If you’re concerned about warranties and customer support, it’s always a good idea to go with a brand you recognize and trust.
Cooling Capacity: 10100 btu/hr
Combined Energy Efficiency Ratio (CEER): 15.7
Annual Energy Use: 482.5 kWh/yr
Cooling Capacity: 10,000 btu/hr
Combined Energy Efficiency Ratio (CEER): 15
Annual Energy Use: 500 kWh/yr
Cooling Capacity: 8000 btu/hr
Combined Energy Efficiency Ratio (CEER): 15.0
Annual Energy Use: 400.0 kWh/yr
Cooling Capacity: 10000 btu/hr
Combined Energy Efficiency Ratio (CEER): 15.0
Annual Energy Use: 500.0 kWh/yr
Cooling Capacity: 10000 btu/hr
Combined Energy Efficiency Ratio (CEER): 15.0
Annual Energy Use: 500.0 kWh/yr
Because window air conditioners have a relatively long life, paying a few hundred dollars more up front to get a unit that’s more energy efficient will have a significant impact on your electricity usage. And, not only do you get a better performing air conditioner when you get an inverter model, you also typically get other luxuries, including the ability to program the air conditioner to cool at certain times of the day and other smart features.
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