When it's hot outside, temperatures indoors soar even higher. Inside activities become a chore and sleeping is especially hard.
One economical way to chill down hot, humid rooms is to install a window unit air conditioner. Window unit A/Cs often come at a price point that's comfortable for many buyers, and they cool and dehumidify only the required space.
But a crucial part of buying a window unit A/C is to correctly size it to the room. If you size an A/C unit too large for the room's needs, you end up paying for cooling capacity that you'll never use and a unit that cycles on and off too much to function efficiently. If you size the unit too small, you'll have rooms that never quite get cool enough—no matter how hard the unit is working.
6 Factors That Affect Cooling Needs
Estimating the conditions of the room that you want to cool, plus its surroundings should be considered, too.
Number and Size of Windows
Between 25- and 30-percent of a home's thermal transference is through its windows. So, if the room that you want to cool down has many windows, or if those windows are large, you'll want to look for a larger-capacity window unit.
A well-insulated room is easier, faster, and less expensive to cool down. Older homes may not be insulated in the walls or ceilings. Rooms of this type will require a larger A/C unit as smaller units will cycle on more frequently or not even be able to keep up with the cooling requirements.
Shade or Sun
When the room is heavily shaded by trees or by other structures, reduce the BTU needs by 10-percent. For rooms that have full sun and no shade, add 10-percent to the BTU needs.
If two or more people use the room, add another 600 BTUs per person.
Window unit A/C recommendations are based on 8-foot ceilings. A 10-foot-high ceiling represents 25-percent room volume, so add another 25-percent to your BTU needs.
Electrical Outlet: Volts and Amps
Some units in lower BTU categories may require standard 120V, 15-amp outlets. Your outlet's amperage rating plays a significant role in your room's cooling. Units that require 230V / 208V 60Hz or 110V / 120V 60Hz with 20-amp outlets cannot be run on 15-amp outlets.
Additionally, you cannot simply change out your 15-amp circuit breaker for a 20-amp breaker if that circuit uses 14 gauge wire (which is rated for 15 amps). You would need to run a completely new circuit using 12 gauge wire.
Window unit A/C cords tend to be about 6 feet long. They cannot safely be extended with extension cords. So, the window and the outlet must be about five feet within each other.
Buying the Right Category of Window A/C Unit
Window unit A/C cooling capacity is measured in BTUs (British Thermal Units). Higher BTUs mean a greater cooling capacity.
The U.S. Department of Energy's EnergyStar division issues BTU recommendations (per hour) based on room square footage space. These recommendations translate to the window unit A/C BTU category that you should buy. Factors such as sun or shade, ceiling height, and occupancy may affect the size of A/C to buy.
|Guide to Window Unit A/C Sizing|
|Room Square Footage||BTU Requirements||Window Unit A/C BTU Category to Buy|
|100 to 150||5,000 to 6,000||5,000 or 6,000|
|150 to 250||6,000 to 7,000||6,000 or 7,500|
|250 to 300||7,000 to 8,000||7,500 or 8,000|
|300 to 350||8,000 to 9,000||8,000 or 9,000|
|350 to 400||9,000 to 10,000||9,000 or 10,000|
|400 to 450||10,000 to 12,000||10,000, 11,000, 11,800, or 12,000|
|450 to 550||12,000 to 13,000||12,000 or 14,000|
|550 to 700||14,000 to 18,000||14,000, 15,000, 17,000, 17,000, or 18,000|
Air Conditioner Cooling Room Guide
Every room of the house serves separate functions. This means that each room experiences different levels of heat. In addition, people using those rooms have different expectations for how cool the room should be.
For small- or medium-size bedrooms, use a window unit A/C in the 5,000, 6,000, or 7,500 BTU category.
Home offices, like bedrooms, tend to be on the smaller end of the size range, 100 to 200 square feet. In fact, many home offices are converted bedrooms. Office equipment such as desktop computers and copiers may generate negligible amounts of heat.
For home offices, use a window unit A/C in the 5,000, 6,000, or 7,500 BTU category. If most of the work happens during the day, prioritize units in the higher ranges.
Main or Larger Bedrooms
Main bedrooms, larger bedrooms, and bedrooms connected to a bathroom have more square footage than their neighboring bedrooms, so a larger A/C is needed.
For large bedrooms in the 200 to 500 square foot range, use a 7,500, 8,000, 9,000, or 10,000 BTU category window unit A/C.
Living rooms are usually the largest room in the house. While most activities in the living room tend to be more sedentary—reading, streaming, or browsing—more people congregate here, which adds to the cooling needs.
For living rooms, use a 7,500, 8,000, 9,000, 10,000, or 12,000 BTU category window unit A/C.
Kitchens generate huge amounts of heat. Convection ovens and stoves are hot, but even microwaves, toasters, and toaster ovens throw off a considerable amount of heat.
At the same time, kitchens tend to be small and they aren't occupied as frequently as other rooms are. Plus, kitchens often don't have the type of window required for a window unit A/C. So, these factors can curb the need for more air conditioning.
For kitchens, use a window unit A/C in the 5,000, 6,000, 8,000, or 9,000 BTU category.
What to Consider Before Buying
- How many rooms do you intend to cool? If you need to cool many rooms, you may want to look into installing central air conditioning.
- What's it like to put in and take out? Window unit A/Cs in some parts of the country should be installed in spring and removed in early fall. Unless you live in a perenially hot area, you'll be moving this unit in and out twice a year.
- Are you overbuying your window unit A/C? While it's tempting to buy a maximum capacity unit, they are expensive, bulky, and louder than lower capacity units.