Window AC unit guide: sizing, installation & top picks

If you live in a house (or even an apartment that allows it), the window AC unit remains one of the most straightforward ways to cool a single room. It sits right in the sill, vents hot air outside, and doesn't take up floor space like a portable model. I've installed and used dozens of these over the years, and they're a great entry point into cooling a space, especially when a full central system isn't an option. Before you buy one, it helps to understand the basics of sizing, installation, and what makes one model better than another for your specific room. For a full comparison of your cooling options, you can read the complete guide to air conditioning types.

For the bigger picture, see our air conditioning types guide.

What is a window AC unit and how does it work

A window air conditioner is a self-contained cooling system designed to fit into a standard double-hung or sliding window. It works by drawing warm room air in through a filter, passing it over cold evaporator coils filled with refrigerant, and blowing the cooled air back into the room. The heat absorbed by the refrigerant is pumped to the outdoor side of the unit, where a condenser coil and fan release it outside.

The key components inside the box are a compressor, two sets of coils (evaporator and condenser), a fan, and a thermostat. Most modern units also have a built-in dehumidification function, as warm air hits the cold coils, moisture condenses and either drips outside or collects in a small tray where the fan helps evaporate it.

I usually tell people that a window AC is a "one-room solution." It's not trying to cool the whole house, and if you try to oversize it for a small room, you'll actually end up with clammy, uncomfortable air because it cycles off too fast to remove humidity. Getting the sizing right is the most common mistake I see.

How to size a window AC unit for your room

Sizing a window AC is about matching the cooling capacity (measured in BTUs) to the room's heat load. Too little capacity and it runs constantly without reaching the set temperature; too much and it short-cycles, leaving the room humid. Here's how I approach it.

The starting point is square footage. A rough rule of thumb I use is:

• 150-250 sq ft: 5, 000-6, 000 BTU
• 250-350 sq ft: 7, 000-8, 000 BTU
• 350-450 sq ft: 9, 000-10, 000 BTU
• 450-550 sq ft: 11, 000-12, 000 BTU

But square footage is just the baseline. You need to adjust for several factors:

• Ceiling height: If your ceiling is over 8 feet tall, add 10-15% to the BTU requirement.
• Sun exposure: A south- or west-facing room with large windows gets significantly hotter. I usually bump up the BTU by about 10-15% for rooms with heavy direct sunlight.
• Room use: A kitchen generates a lot of heat from cooking appliances, so it may need an extra 4, 000 BTUs. A shaded bedroom with good insulation may need slightly less.
• Number of occupants: Each additional person beyond two adds about 600 BTUs.

Another mistake I see is people buying a unit that is too large for a bedroom because they want it to cool fast. That backfires. A properly sized unit runs longer, removes more humidity, and keeps the temperature more consistent. When in doubt, it is better to size slightly small than slightly large for a bedroom, especially in humid climates.

Key features to consider when choosing a window AC

Not all window ACs are the same. After installing and testing many models, there are a few features that really matter for comfort and convenience.

Energy efficiency ratings (EER and CEER) tell you how much cooling you get per watt of electricity. The higher the rating, the cheaper it is to run. Most modern units have CEER ratings between 10 and 14. I generally recommend choosing a unit in the top third of the range for your size category, the upfront cost is a bit higher, but it pays for itself within a couple of seasons.

Noise level is a big deal if the unit goes in a bedroom. Look for a model with a "sleep mode" that runs the fan slower at night. The lowest noise ratings I've seen on quieter units are around 42-50 decibels, which is about the sound of a quiet conversation. I usually test a unit in the store by standing next to it, if the hum bothers me there, it'll bother me at 2 AM.

Remote control and smart capabilities are useful but not essential. A basic remote is fine for changing temperature from across the room. Smart features (Wi-Fi, app control, voice assistant) let you schedule operation or turn it on before you get home. I find them most useful in a living room where you might want to cool the space before you arrive.

Filter type matters for air quality. Most window units have a washable foam or mesh filter that you should clean every two weeks during heavy use. Some higher-end models include a permanent electrostatic filter that captures smaller particles. I always recommend checking that the filter is easy to remove and clean, some designs require disassembling the front grille, which gets annoying fast.

Installation flexibility is another consideration. Some units have a slide-out chassis that makes seasonal removal and reinstallation easier. Others are designed to fit narrower windows or casement windows. Always measure your window opening width and height before you buy, nothing is worse than getting a unit that doesn't fit.

Top window AC unit brands and models compared

I've worked with most of the major brands over the years, and each has strengths depending on what you need.

LG consistently produces quiet, energy-efficient units with good smart features. Their dual-inverter models are noticeably quieter than standard compressors, and the CEER ratings are usually among the highest. They tend to be a bit pricier, but the build quality is reliable. I've seen many LG units last 8-10 years with regular filter cleaning.

Frigidaire is a solid mid-range choice. Their Gallery series offers a sleek, flush-mount design that looks better than most window units and includes smart controls. They aren't the absolute quietest, but the price-to-performance ratio is excellent. I've installed several Frigidaire units in rental properties and they hold up well.

Midea has been gaining ground with their U-shaped inverter window AC. The design is unique, it hangs partially outside the window, which allows the window to close down over the unit, reducing noise and allowing natural light. The variable-speed compressor makes them very quiet and efficient. Installation is a bit different from a traditional unit, but the results are impressive.

GE makes good basic units at competitive prices. Their AHY series is simple, reliable, and easy to install. You don't get smart features or the lowest noise level, but you get a dependable workhorse. GE units also tend to have a straightforward installation kit that includes accordion panels and foam seals.

LG also makes good budget models, but I prefer to spend a little more on their inverter line. In general, for a bedroom, I lean toward Midea for the quietest operation. For a living room, LG or Frigidaire offer a good balance of features and durability.

Step-by-step window AC installation guide

Installing a window AC correctly makes a huge difference in performance and safety. Here's the process I follow.

What you'll need

• Window AC unit
• Screwdriver (Phillips and flathead)
• Measuring tape
• Level
• Foam weather stripping or the included seal
• L-brackets or support brackets (for heavy units or upstairs windows)
• Optional: caulk or silicone for sealing gaps

Steps

1. Prepare the window. Open the sash enough to fit the unit -- usually 13-16 inches for most models. Clean the sill and remove any old weather stripping or debris. Make sure the window tracks are clear.

1. Install the side curtains. Most units come with accordion-style filler panels. Slide them into the channels on either side of the unit. They expand outward to fill the gap between the unit and the window frame.

1. Position the unit. Lift the AC into the open window and center it on the sill. The unit should tilt slightly downward toward the outside (about a 1/4-inch drop) so condensation drains properly. Use a level to check the tilt.

1. Close the sash. Lower the upper window sash so it rests on top of the unit. Some units have a bracket or screw that holds the sash in place. Tighten any included locking screws.

1. Extend the side curtains. Pull the accordion panels outward until they touch the window frame. Secure them with the included screws or clips. Do not overtighten -- you just need them snug.

1. Seal gaps. Stuff foam weather stripping into any gaps between the unit and the window frame, especially around the side curtains and the gap between the sash and the unit. This keeps hot air out and cool air in.

1. Secure the unit. For heavy units (over 50 pounds), install L-brackets or a window support bracket on the outside, anchored to the window sill. This prevents the unit from falling out. For upper-floor installations, this step is not optional.

1. Plug it in. Use a dedicated outlet if possible. Never use an extension cord with a window AC, as the high current draw can cause overheating.

One tip I've learned from experience: before you install, check that the window sill is level and strong enough to support the weight. Some old sills rot or sag, and a heavy unit can cause the entire window frame to shift.

Window AC vs portable AC: which is right for you

People often ask me whether a window unit or a portable AC is a better choice. Here's the honest breakdown.

A window AC is almost always more efficient than a portable unit of the same BTU rating. The portable's exhaust hose sits inside the room, and it draws some cooled room air to vent the hot exhaust outside, creating negative pressure that pulls hot air in from other parts of the house. Window units don't have this problem because the compressor and condenser are already outside the room.

Window units also tend to be quieter when running, because the compressor noise is partly muffled by the window. Portable units have the compressor inside the room, right next to you.

However, portable units win on installation flexibility. They don't require a window that opens vertically; they work with sliding windows, and some even work with casement windows using an adapter kit. They're also easier to move from room to room on casters.

For cost, a window unit typically runs $150-$500, while a portable of similar capacity is $300-$700. I generally recommend a window unit if you have a suitable window and don't need to move it often. Go portable only if you can't use a window (HOA restrictions, apartment rules, or window design) or need to cool multiple rooms at different times.

Window AC maintenance and energy-saving tips

A window AC doesn't need much maintenance, but a little goes a long way in terms of efficiency and lifespan.

Clean or replace the filter every two to four weeks during peak cooling season. A dirty filter can reduce airflow by 15-20%, making the unit work harder and use more electricity. Most filters are washable with warm water and a mild detergent. Let them dry completely before reinstalling.

Check the seal around the unit a few times during the summer. The foam strips can degrade or get knocked loose, especially if the window is opened and closed. Use fresh weather stripping if you notice drafts.

Cover the unit in winter if you leave it in the window. Use a heavy-duty AC cover that fits snugly over the exterior. This prevents snow, ice, and cold air from damaging the internal components. Do not cover the unit while it's running, covers are for storage only.

Use a timer or smart control to avoid running the AC when no one is home. Setting the unit to turn on 30 minutes before you arrive keeps the room comfortable without wasting energy during the day. Most units with a remote have a 1-24 hour timer built in.

Keep the area around the unit clear, pull furniture away from the front and sides to allow good airflow. Blocked airflow is one of the most common causes of an AC freezing up or running inefficiently.

For details on installation costs and hiring a contractor, see our HVAC installation cost guide.

Frequently asked questions about window AC units

How long does a window AC unit typically last?

With proper maintenance, regular filter cleaning and seasonal storage, a good-quality window AC should last 8 to 10 years. Lifespan depends on how many months per year you run it and how well you protect it from weather extremes.

Should I cover my window AC in the winter?

Yes, but only if the unit stays in the window. Use a fitted, weatherproof cover over the exterior portion. This prevents water, ice, and debris from damaging the coils and fan. Never run the unit while the cover is on.

What should I do if my window AC freezes up?

Turn the unit off and let it thaw completely. A frozen coil is usually caused by low airflow (dirty filter or blocked vents) or running the AC in very cool outdoor temperatures. Check the filter first, it's the most common cause. If the problem persists, the refrigerant charge might be low, and you need a professional.

Can I use an extension cord with a window AC?

Generally, no. Window ACs draw a lot of current, and extension cords can overheat. Always plug the unit directly into a grounded wall outlet. If you must use an extension cord, use a heavy-duty 14-gauge or thicker cord, but you're better off avoiding it altogether.

Which brand is the quietest for a bedroom window AC?

Midea's U-shaped inverter models are consistently rated the quietest, with noise levels around 42 decibels on low. LG's dual-inverter models are also very quiet. Avoid budget models with fixed-speed compressors if noise is a primary concern.

Do I need a circuit breaker for a window AC?

A standard 15-amp circuit can handle most window ACs (up to about 12, 000 BTUs). If your unit draws over 12 amps, it should be on a dedicated circuit. Check the unit's label for the required amperage and avoid plugging other heavy appliances into the same circuit.