What should be on a window AC buying checklist?

Measure the room before you compare units

The first item on a window AC checklist is a tape measure, not a product page. Measure the length and the width of the room in feet, converting inches to fractions of a foot (9 inches is .75 ft, 6 inches is .5 ft). Multiply the two numbers to get square footage. A room 14 ft 9 in long and 10 ft wide works out to 147.5 sq ft.

Odd shapes are still workable. For a triangular area, multiply length by width and divide by two. For an L-shaped or otherwise complex room, break it into squares, rectangles and triangles, work out each piece, then add them together. Choosing the right BTU for your room size walks through the same arithmetic with more examples.

Turn square footage into a cooling capacity

With the area known, look up the capacity the room needs. The chart below is the one ENERGY STAR publishes; capacities are in BTU per hour and assume an 8-foot ceiling.

Adjust the figure for sun, shade, people and cooking

The chart figure is a starting point. ENERGY STAR lists four adjustments: reduce capacity by 10 percent for a heavily shaded room, increase it by 10 percent for a very sunny room, add 600 BTU for each person beyond two who regularly occupy the room, and add 4,000 BTU if the unit goes in a kitchen.

Then round down to the nearest thousand and look for that number on the listing. Rounding down rather than up is deliberate: the next size up usually carries more capacity than the room needs, and extra capacity brings its own problems.

A room that is hard to cool for reasons the chart cannot see, such as a large west-facing window or thin insulation, is covered in why some rooms are harder to cool than others.

Why an oversized window unit is a poor choice

A larger unit cools the air faster, but lowering the temperature and drawing moisture out of the air are not the same job. ENERGY STAR explains that an oversized unit brings the room down to temperature before moisture has been drawn out of the air, so the room stays cool and moist and feels damp and clammy. The Department of Energy's federal purchasing guidance makes the same point, noting that oversizing leads to excessive on-off cycling and poorer moisture removal.

So the checklist rule is to buy the capacity the room calls for, not the largest unit the window will hold. What happens when a room AC is too big covers the symptoms to expect when the match is wrong.

Read the CEER figure, then check the ENERGY STAR level

CEER is the combined energy efficiency ratio used for room air conditioners: a higher number means more cooling for the same energy input. CEER explained covers how the rating is put together.

ENERGY STAR sets base CEER levels that depend on capacity band and on whether the unit has louvered sides, with separate levels for casement types:

What the CEER spread looks like in practice

For a sense of the range, the Department of Energy's federal purchasing page uses a 12,000 BTU/h example in the 8,000 to 13,999 BTU/h band with louvered sides. The highest efficiency model listed there reaches 15.4 CEER, the ENERGY STAR level is 14.7, and a less efficient model sits at 10.9.

That gap is the reason the efficiency line belongs on the checklist before the feature list. Two units can carry the same capacity figure and sit at opposite ends of that range. What ENERGY STAR means for a room air conditioner explains what certification adds beyond the number itself, including tighter side panels and clearer sealing instructions.

Variable speed compressors change how the unit runs

In a conventional room air conditioner the compressor behaves like an on/off switch: it runs at maximum capacity when the thermostat calls for cooling and stops when the set temperature is reached, alternating between the two. A variable speed compressor varies the motor speed instead, so it can hold a temperature continuously.

ENERGY STAR states that most certified room air conditioners include variable speed technology, and that the approach allows quieter cooling with significantly less energy use. If a listing names an inverter compressor, that is the same idea under a different label. Inverter vs fixed-speed room air conditioners sets out the difference in run behavior.

Confirm the unit fits your window type

A window AC has to match the window, not just the room. Double-hung windows are the common case. Casement and slider windows need a casement-only or casement-slider unit, which ENERGY STAR rates on its own CEER levels. Which window types take a window air conditioner goes through the fit questions, including frame depth and storm windows.

Shape is the other half of the fit. A standard unit sits on the sill and blocks part of the window. U-shaped and saddle-style units bracket the window or sill instead, so the window can still open and close, and in most cases the compressor sits outside the room while the control panel and vent stay inside. ENERGY STAR notes that these designs reduce noise and keep the view clearer. Standard vs U-shaped window air conditioners compares the two layouts.

Plan the install, because sealing is part of the purchase

ENERGY STAR puts the cost of a poor fit in concrete terms: gaps around a badly installed unit let in as much air as a six-square-inch opening in the wall, which raises energy use and makes the room less comfortable. That makes the side panels and insulation in the box part of the product rather than an afterthought: follow the instructions and use the insulation materials supplied with the unit.

Two more install items belong on the checklist. The unit must be level so the drainage system works effectively, and airflow direction matters if the unit sits near a corner of the room, so look for adjustable louvers. Sealing the sides of a window AC and L brackets and support brackets cover the parts that keep the unit steady and the gaps closed.

ENERGY STAR also suggests removing the unit at the end of the cooling season where possible, or fitting an appropriately sized cover, to limit heat loss through the window.