With the dog days of summer just around the corner, the perfect time to shop for a new air conditioner is now. There are numerous factors to consider when you’re shopping for an air conditioner, including whether your home can accommodate a ductless mini-split system, a built-in wall assembly, a window unit, or a portable model. But all air conditioners have one thing in common: They must be sized properly in order to effectively lower the temperature and remove excess moisture from the air, resulting in a comfortable, cool indoor environment. When you’re shopping for an appropriately sized air conditioner, a number of factors come into play, including the general climate and average summer temperatures in your region of the country; the square footage of the room or rooms to be cooled; the installation location in the wall, window, or ceiling; the number of people typically occupying a room; and the amount of insulation in the home. An air conditioner that is too small for a given area will not be able to cool the space efficiently, while an air conditioner that is too large will tend to cycle on and off too rapidly, wasting energy and impairing the unit’s ability to remove humidity from the room.

Air conditioners are rated by their seasonal energy efficiency ratio (SEER), and their capacity is expressed in British thermal units (BTUs). The BTU rating gives an indication of how quickly and effectively a particular unit can cool the room where it is located. Most home air conditioners sold in the United States range from around 5,000 BTUs to more than 20,000 BTUs.running fan on ac unit“Since we measure all of our heating and air-conditioning units in BTUs, it’s important to know how many BTUs your living space requires so you don’t buy a unit that is too large or too small. ac power supply repairAnother common measurement that heating and air-conditioning units are assigned is ‘tonnage.’ air conditioner and heater in one unit

But don’t let this confuse you—one ton is just 12,000 BTUs!” Correctly sizing an air conditioner for a given room requires a tape measure and a few simple calculations. The first step is to determine the size of the room where the unit will be installed. offers a handy sizing calculator to determine the correct amount of BTUs needed for a given room or set of rooms. The calculator requires the dimensions of a room, in length and width; the type of room, such as kitchen or bedroom; the number of people typically in the room; and finally, the exposure of the room—whether it is very sunny or shaded. Once you plug in the appropriate information, the calculator will determine the correct size air conditioner in BTUs. If you are cooling two adjacent areas, or if your room is odd-shaped, determine the square footage of each space as if it were a separate room, and then add the two measurements together to get a total amount of BTUs. According to the U.S. Department of Energy, another rating that homeowners should consider when purchasing a new room air conditioner is the Energy Efficiency Rating, or EER, which represents the cooling capacity of a unit in BTUs per hour divided by the watts of power consumed at a specific outdoor temperature (usually 95 degrees Fahrenheit).

The EER rating is found on the yellow Energy Guide label on the air conditioner, and it typically ranges from 8 to 11.5. An EER rating of 10 or higher is the most efficient and will yield the highest savings on monthly electric bills. A higher EER also helps the environment by reducing greenhouse emissions. offers a large selection of mini-split air conditioners and accessories from the top manufacturers in the industry, and features a variety of informative tools and instructional videos on its Web site. How many Btus is recommended for a room about 35 square meters? I have 2 units with 13,000 Btus each. I'm thinking it should be enough, but the room is just so-so cool. I can never say it's COLD in the room. This one is for a single room. Assume the room direction is for the northern hemisphere. Reverse North and South if you're in the south. From the comment by mike: Consumer Reports groups single-room A/Cs roughly into small/medium/large categories (100 to 300 square feet, 250 to 400 square feet, 350 to 650 square feet), and from what I've seen the BTU ratings and room sizes on A/C boxes in stores seem to follow the same general formula (probably they're all referencing the Energy Star calculations).

Yes, one can quibble about floor space versus room surface area, but using the former as proxy for the latter seems to work well enough for most of the rooms in most of our houses. However they note that Energy Star suggests you make some adjustments: If more than two people regularly occupy the room, add 600 BTUs for each additional person. (A human puts out roughly 100W of heat.) Another factor: Energy efficiency. EER is the energy efficiency rating while under continuous operation; CEER is the rating with "energy saver" engaged (and automatic fan speed control, if the unit supports that). Energy saver modes try to turn the compressor and/or fan off when the target temperature has been hit, and turn the A/C back on only when the room has drifted a few degrees away from that temperature. The fluctuation is noticable and some find it annoying... but it does significantly reduce operating cost. I went up a size on the unit I just purchased because the smaller one (a) was right on the edge of being able to handle the room size I needed without allowing for solar heating, and (b) had a CEER about 10% lower.

Going up that step did increase the price from $120 to $180, but it was well within my budget either way. The biggest advantage of the small one would have been that it would fit in the narrow dormer window rather than the standard-width end-wall window. (Now I need to decide whether it's time to replace the big one that covers most of the entertaining area. I try not to run that unless I must, so maybe I'll stick with the older and less-efficient for another year or two. Unless it makes the decision easy by dying.) For what it's worth, GE's base recommendation (from the packaging) is: 150 sq ft: 5000 BTUAll residential air conditioners will carry a label showing the unit's cooling output in kilowatts (kW). Every room and every home is different, so we recommend that you get a licensed contractor to visit your home and provide a detailed heat load calculation. Using the calculator below, however, will give you a rough idea of the cooling output you will need: 1. Please select your nearest regional

2. What is the ceiling area of the room you wish to Uninsuladed metal roof overInsulated metal roof overUninsulated tiled roof overInsulated tiled roof overUnairconditioned room over with carpeted floorsUnairconditioned room over with no carpets 3. What is the floor area of the room you wish to Uninsulated suspended timber floor enclosed underInsulated suspended timber floor enclosed underUninsulated suspended timber floor unenclosed underInsulated suspended timber floor unenclosed underUninsulated suspended concrete slab enclosed underInsulated suspended concrete slab enclosed underUnairconitioned room below is the height of the ceiling in the room you are cooling? is the total area of the internal walls with unconditioned space adjacent? is the total area of external walls including all the windows? Weatherboard with no insulationWeatherboard with insulationCement and metal sheet with no insulationCement and metal sheet with insulationBrick veneer with no insulationBrick veneer with insulationConcrete block masonry with no insulationConcrete block masonry with insulation