What to Check Before You Buy a Window Air Conditioner

The essentials

1. Size the unit to the room: 5,000 to 6,500 Btu cools 100 to 250 square feet, and 9,800 to 12,500 Btu covers 350 to 550 square feet.
2. An oversized unit cools the air before it removes humidity, leaving the room damp and clammy while wasting energy and costing more to buy.
3. ENERGY STAR certified room air conditioners use 9 percent less energy than standard models and average less than $70 per year to run.
4. Use the sealing and insulation materials that come with the unit; a poorly installed room AC can leak as much air as a 6 square inch hole.
5. Search the CPSC recall database by model number before you buy, and register the product after purchase so you hear about recalls.

A window air conditioner can cool a single room for far less than a central system, but only if you buy the right one. A unit that is too small runs constantly and never quite gets there. One that is too big chills the air so fast it never pulls out the humidity, leaving the room cold and clammy. This checklist covers what to measure, check and compare before you spend money.

Measure the window and the room first

• Window type. Standard window units are made for double-hung windows that open by sliding up from the bottom. If you have casement or slider windows, you need a model built for those, so confirm the window style before you shop.
• Window width. Measure the opening where the unit will sit and compare it to the manufacturer’s listed dimensions, including the side panels that fill the gap.
• Room dimensions. Multiply the length by the width. If two spaces are not separated by a door, add their square footage together, because the unit will need to cool both.
• Ceiling height. The standard sizing charts assume 8-foot ceilings. If your ceilings are taller, you may want a unit with more cooling capacity.
• Sill and frame condition. Look for a rotten sill, loose frame or a storm window in the way. Any of these can complicate the install or keep the unit from seating and sealing properly.
• The outside of the window. Make sure there is a safe place for the back of the unit to sit and drain, and that nothing blocks airflow out of the grille.

Renters should also read the lease or ask the landlord before buying. Some buildings restrict window units or require specific support brackets.

Match the cooling capacity to the room size

Cooling capacity is measured in British thermal units (Btu) per hour. The chart below, based on ENERGY STAR sizing guidance and Consumer Reports’ buying advice, matches capacity to room size:

Then adjust for the room itself:

• Very sunny room: increase capacity by 10 percent.
• Heavily shaded room: reduce capacity by 10 percent.
• More than two people regularly in the room: add 600 Btu for each additional person.
• Unit used in a kitchen: add 4,000 Btu.

If your room has an unusual shape, a sales associate can help you work out the square footage. Resist the urge to size up “just in case.” An air conditioner removes both heat and humidity, and a unit that is too big shuts off before it dehumidifies, so you end up with cool, moist air that feels damp. Bigger also means a higher price and higher energy use with no better cooling.

Check your electrical outlet and circuit

• Check the plug on the model you are considering. Manufacturers list the electrical requirements in the specifications and manual. Compare them to the outlet in the room before you buy, not after.
• Do not plan on an extension cord. Plug the unit directly into a wall outlet as the manual directs.
• Think about what else is on the circuit. A window unit sharing a circuit with other appliances can trip the breaker, so note what else runs on that outlet’s circuit.
• Call a licensed electrician if the outlet does not match. Upgrading an outlet or circuit is not a do-it-yourself job. An electrician can confirm what the wiring can handle and make any changes safely.

Read the yellow EnergyGuide label

• Find the CEER rating. That stands for Combined Energy Efficiency Ratio, and higher numbers mean the unit delivers more cooling per unit of electricity. Use it to compare models in the same size class.
• Check the estimated yearly energy cost. ENERGY STAR certified room air conditioners cost less than $70 per year to run on average, and they use 9 percent less energy than standard models.
• Weigh the price difference against the savings. The Department of Energy’s Federal Energy Management Program has calculated that an ENERGY STAR qualified room air conditioner pays for itself if it costs no more than $167 above a less efficient model, in 2023 dollars, with the best available model saving up to $189. Those figures are based on a 9-year lifetime of energy costs.
• Look for the ENERGY STAR label itself. Manufacturers meeting the program’s efficiency requirements are allowed to display it, and the ENERGY STAR website lists certified room air conditioners.

Compare features worth having, and skip the rest

• Variable speed compressor. In a conventional unit, the compressor cycles fully on and off to hold the temperature. A variable speed compressor changes its output continuously, which cools more quietly and saves a significant amount of energy. Most ENERGY STAR certified room air conditioners now include it.
• U-shaped or saddle design. These units bracket the window or windowsill instead of sitting on top of it, so you can still open and close the window. The controls and vent stay inside while the noisy compressor sits outside, which cuts noise and keeps the view clear.
• Connected controls. Smart models can show feedback on the unit’s energy use, and many are smart-grid ready, which may let you enroll in energy bill savings programs some local utilities offer.
• Sealing materials and clear instructions. ENERGY STAR certified models come with better materials and clearer directions for sealing and insulating around the unit, which minimizes air leaks.
• A filter you can reach and clean. You will be cleaning it regularly, so check how it comes out.

Features to treat as nice-to-haves rather than deciding factors: remotes, apps and extra modes. Paying for capacity you do not need to get them is a poor trade.

Check for recalls before you pay

• Search the model number on the CPSC website. The Consumer Product Safety Commission posts recalls for window and room air conditioners. It recently recalled about 1.7 million U and U+ window units sold under brand names including Midea, Frigidaire, Insignia and several others, because pooled water that did not drain quickly enough could lead to mold growth.
• Know the remedy if a unit you own is recalled. In that recall, affected owners could get a free repair or a full or prorated refund. Buying a current model from a retailer makes any future remedy easier to claim.
• Register the product after purchase. Registration is how the manufacturer reaches you if a recall is announced later.

Plan the installation before the unit arrives

• Check the shipping weight. Larger window units are awkward and heavy. Plan on a second person to help carry and set it, and clear the path beforehand.
• Plan the lifting. Setting a unit in a window often means working from a ladder outside. Review our ladder safety basics before you start, and skip the outside work entirely in bad weather.
• Use a support bracket if the manual calls for one. Follow the installation instructions, and check whether your landlord or building requires a bracket for units on upper floors.
• Seal every gap. Use the insulation panels and foam that come with the unit. An improperly installed room air conditioner can leak as much air as a 6 square inch hole, which wastes money all summer.
• Tilt it as the instructions describe. Most units need a slight backward tilt so condensate drains outside rather than into the room.
• Decide on the winter plan now. In cold months, either remove the unit or insulate it from the outside with a tight-fitting cover so heated air does not escape. Our fall home maintenance checklist includes this job along with the rest of the season’s work.

If your home’s central system also cools the house, keep it running well with regular filter changes. Our HVAC air filter guide covers sizes, MERV ratings and how often to change them.

When a window unit is not the right choice

• Casement or slider windows without a compatible model. Casement-only and casement-slider units exist, but not every room or window accepts a standard unit.
• You need to cool most of the house. A window unit is made for one room. If you are cooling the whole home, a central air conditioner or a ductless mini-split heat pump is the better tool, and a contractor can tell you whether your home is a good fit.
• The room is always too hot or too cold. ENERGY STAR points out that rooms with temperature problems may have inadequate air sealing, windows or insulation. A new window unit will fight those problems instead of fixing them, so look at sealing and insulation first.

Related guides:

• HVAC Air Filters: Sizes, MERV Ratings and When to Change Them: Useful if you run central air alongside a window unit, or want cleaner air in the cooled room.
• Ladder Safety for DIY Jobs Around the House: Installing a window unit usually means ladder work on the outside of the house.
• A Fall Home Maintenance Checklist: Covers removing or covering the unit and other end-of-season jobs.

Common questions

What happens if I buy a window air conditioner that is too big for the room?

A unit that is too large cools the room quickly and shuts off before it removes much humidity, so the space feels cold but damp. It also costs more up front and wastes energy without cooling any better.

Do window air conditioners need a special outlet?

Requirements vary by model and capacity, so check the plug type and electrical requirements listed in the manufacturer’s specifications before you buy. If your outlet does not match what the unit needs, have a licensed electrician look at the circuit rather than using an adapter.

How do I find out if an air conditioner has been recalled?

Search the model number on the Consumer Product Safety Commission’s website before you buy. For example, the CPSC announced a recall of about 1.7 million U-shaped window air conditioners sold under several brand names because pooled water could cause mold growth.

Should I leave the unit in the window over the winter?

Either remove it in the cold months or cover it from the outside with a tight-fitting air conditioner cover so heated air does not escape. Either way, the unit should fit tightly in the window while it is in use so outdoor air stays out.

This guide is general information, not professional advice. Follow the manufacturer’s instructions and your local building codes, and hire a licensed professional for electrical, gas, structural or roofing work.