The most energy-efficient air conditioner for your window will depend on the size of your room and the energy efficiency rating of the model.
Window air conditioners are more energy-efficient than central air conditioners, but air conditioners will always need and use a lot of power.
To find the most efficient air conditioner for your window, you need to compare energy efficiency ratings of appropriately-sized models to find the best one.
Are window air conditioners energy-efficient?
Window air conditioner models are a great low-cost and high-efficiency alternative to expensive central air systems and high-maintenance portable air conditioning units. However, these units, in themselves, are not particularly energy-efficient.
Air conditioners use a lot of power to start up and run, and even though window air conditioners are usually smaller and use less electricity than central air or HVAC systems, they are still not very eco-friendly appliances to use.
Most efficient air conditioners for your window
This means that when you are considering purchasing a new window air conditioner, it is important to try and find the most efficient models, so that you can be as environmentally conscious as possible, while still ensuring that your space is cool enough for you to comfortably work, sleep, or simply live in when the outside temperatures are high.
Finding a very energy-efficient air conditioning model can also save you money on your electricity bill in the long run, so it is worthwhile to look around to find the best one.
Fortunately, there are various standardised ratings that can help you compare the energy efficiency of window air conditioner models from different brands.
These ratings included the Energy Efficiency Ratio (EER rating), the Seasonal Energy Efficiency Ratio (SEER rating) and the Combined Energy Efficiency Ratio (CEER rating), all of which can be summarised as follows:
| Type of rating system | Minimum rating for energy-efficient models |
| EER |
|
| SEER |
|
| CEER |
|
These rating systems can be confusing, but to start with, window air conditioner models with an Energy Star label have already passed the standards set out by the U.S. Department of Energy (DOE) in terms of energy efficiency and should be quite cost-effective to operate.
You can then start comparing different models of the same size and capacity using any of the above rating systems, as long as you compare ratings from the same category.
It is important that you find a window air conditioner model that is the appropriate size and capacity for your space, as this is the best way to ensure that your AC is highly energy efficient.
Explaining EER, SEER, and CEER ratings
Although the varying rating systems when it comes to the energy efficiency of air conditioners can be confusing, the most important thing to remember is to compare the EER rating to EER rating or CEER to CEER, from one unit to the next.
SEER ratings are only used in larger, central air systems, so you should not need to worry about this rating when it comes to smaller window AC units.
The difference between EER and CEER is simply that the EER rating is measured at a constant inside and outside temperature, while the CEER rating is used by the DEO and it takes the idle power usage into account.
However, both of these ratings should give you a constant to compare different models with, it does not matter too much which one you use.
Most energy-efficient window air conditioners for small spaces
The smallest window air conditioner models are about 6000 BTU, for rooms that are up to 250 square feet in size. In this case, a CEER-rating of over 11 would be considered an energy-efficient model.
Some of the most energy-efficient models in this category include:
| Window air conditioner model | BTU | Energy Star label | Price | CEER rating |
| LG LW6019ER Rated Window Air Conditioner | 6000 BTU | Yes | $279.99 | 12.4 |
| Haier ESAQ406T Serenity Window Air Conditioner | 6000 BTU | Yes | $329.99 | 12.1 |
| BLACK+DECKER BD06WT6 Window Air Conditioner | 6000 BTU | Yes | $229.00 | 12.1 |
| Keystone KSTAW06CE Energy Star Window-Mounted Air Conditioner | 6000 BTU | Yes | $289.99 | 12.1 |
Most energy-efficient window air conditioners for larger spaces
For rooms that are between 300 and 800 square feet, a CEER-rating of 11.2 or higher would be considered very energy-efficient, and some of the best models in this category are:
| Window air conditioner model | BTU | Energy Star label | Price | CEER-rating |
| GE AHEK08AC Window Air Conditioner | 8000 BTU | Yes | $365.00 | 15.7 |
| TOSOT Window Air Conditioner | 10 000 BTU | Yes | $360.00 | 12 |
| LG LW1517IVSM Dual Inverter Smart Window Air Conditioner | 14 000 BTU | Yes | $549.99 | 14.7 |