How to Choose the Right Size Through-The-Wall Air Conditioner for Your Space

Introduction: Getting the Right Size Air Conditioner Matters

Hey, Tony Marino here — and I’ve seen my fair share of homeowners struggle with undersized or oversized air conditioners. Sizing your air conditioning unit correctly is crucial. Get too small a unit, and it’ll run all day without cooling your home properly. Go too big, and it’ll cycle on and off rapidly — wasting energy and leaving you with humidity issues. Plus, improper sizing can shorten your unit’s lifespan.

When it comes to through-the-wall air conditioners, which are permanently installed into your exterior wall rather than a window, picking the right size is even more critical. Once installed, they’re not going anywhere easily. So let’s do this right the first time.

In this article, I’ll help you:

✅ Accurately figure out the BTUs you need
✅ Consider key factors like insulation, ceiling height, and windows
✅ Balance energy savings and comfort
✅ Pick a unit that meets all your cooling goals

Let’s jump in.

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💡 What Does "Sizing" Even Mean for Air Conditioners?

When HVAC pros talk about the “size” of an air conditioner, they mean its cooling capacity — measured in BTUs (British Thermal Units) per hour. The BTU rating tells you how much heat a unit can remove from your space. Generally:

• A higher BTU rating = larger cooling capacity

• A lower BTU rating = smaller cooling capacity

Here’s a quick rule of thumb most homeowners use as a starting point:
20 BTUs per square foot of living space.

That means a 500 sq. ft. room would need roughly a 10,000 BTU air conditioner.

But wait — don’t go buying based on square footage alone. That’s a great first pass, but there are other factors you need to dial this in properly.

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🏠 Factors That Impact Air Conditioner Sizing

🔵 1. Square Footage of the Space

Your first step is knowing the exact area you want to cool. Measure the length and width of the room and multiply to get square footage.

*Figures based on standard 8-ft ceilings and moderate insulation.

If you want an official reference chart straight from the source, check the U.S. Department of Energy’s cooling size calculator. That calculator will help you fine-tune your estimates.

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🔵 2. Ceiling Height

Those BTU guidelines assume standard ceilings (~8 feet). But what if your ceilings are taller? Hot air rises, so extra ceiling height can mean extra heat to remove.

Quick Rule: Add 10% BTUs for every extra 1-2 feet of ceiling height.

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🔵 3. Insulation and Air Leakage

Is your home well-sealed and insulated? If your walls and windows leak like a sieve, your AC will have to work much harder. Poor insulation can add another 10-20% BTUs to your calculation. Conversely, if you have brand-new windows and good insulation, you may be fine going slightly smaller.

If you’d like to read up on how to improve your home’s insulation to support better AC efficiency, this article from Green Building Advisor offers some great pointers.

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🔵 4. Sun Exposure

If the room you’re cooling faces south or west and gets a lot of direct sun, add another 10-15% BTUs. Conversely, shaded or basement rooms may require less.

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🔵 5. Number of Occupants and Appliances

Every human body adds about 600 BTUs/hour, and appliances like ovens or computers add heat too. If the room is often packed with people or contains heat-generating appliances, add 500–1,000 BTUs per appliance or per extra person.

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🧮 How to Do the Math — Step by Step

Let’s do a concrete example so you can follow along.

Imagine you want to cool a 400 sq. ft. family room that faces south, with standard ceilings and moderate insulation. Two adults and a computer setup will be using the space regularly.

Base BTU: 400 sq. ft. × 20 BTU/sq. ft. = 8,000 BTU
Add for sunlight: 8,000 BTU + 10% = 8,800 BTU
Add for people: 2 adults = 1,200 BTU = 10,000 BTU
Add for appliances: computer setup = 500 BTU = 10,500 BTU

So you’d want a unit rated around 10,500 BTUs. Since most through-the-wall ACs come in standard sizes (10K, 12K, etc.), I’d recommend choosing a 12,000 BTU model for a little extra buffer.

If you’d like to explore more room-specific BTU estimates for your home, check out this BTU calculator from PickHVAC — it’s a quick and easy interactive tool.

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💡 Energy Efficiency — Don’t Ignore the EER and SEER Ratings

BTU capacity is important, but so is efficiency. That’s measured by the EER (Energy Efficiency Ratio) or SEER (Seasonal Energy Efficiency Ratio). A higher EER or SEER means the unit will cost less to run.

Here are some things to look for:

• EER ≥ 10.5 is a solid baseline for most through-the-wall units.

• Look for the ENERGY STAR label — especially in newer models that use eco-friendly refrigerants like R-32.

• Consider variable-speed compressors if you can; they adapt output as needed and cut down on energy use.

For more on how refrigerants and AC design impact energy consumption, you can read this informative article by the AHRI, the trade group for air conditioning and refrigeration manufacturers.

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🏠 Installation Concerns That Impact Sizing and Performance

When you choose a through-the-wall air conditioner, the placement matters. A few pointers:

✅ Install at mid-wall height — Hot air rises, so mounting your unit a bit higher can help it draw heat more efficiently.

✅ Make sure the unit is properly sealed. Even a small gap around the unit can let warm, humid air creep in and reduce your AC's efficiency.

✅ Check for obstructions outside. Bushes or walls too close to the exterior part of the unit can restrict airflow.

If you need detailed guidance on installation best practices, the EPA's Section 608 refrigerant handling page also has useful guidance about best practices for servicing and refrigerant safety.

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⚡ Oversized vs. Undersized — What’s the Big Deal?

People often think, “Bigger is better,” but that’s not the case with ACs. An oversized unit:

• Rapidly cools the air but doesn’t run long enough to dehumidify.

• Cycles on and off frequently, which wears the unit faster and wastes energy.

An undersized unit:

• Runs continuously without reaching the target temperature.

• Drives up utility bills because it never catches up.

• Wears out faster due to continuous operation.

In short, the golden zone is choosing a unit that matches your heat load as closely as possible. When in doubt, ask a pro or use one of the many credible BTU calculators like those at energy.gov to double-check your math.

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💰 Balancing Upfront Cost and Long-Term Efficiency

Higher-BTU units cost more up front. But choosing the most efficient model can pay you back with lower electric bills year after year. Prioritize models with:

• ENERGY STAR certification

• R-32 refrigerant, which improves heat transfer and is environmentally friendly

• Adjustable fan speeds and sleep or eco modes to optimize usage overnight

If you’re looking for rebates and incentives for upgrading to a high-efficiency model, check your state’s utility programs or browse the DSIRE database of energy incentives for rebates that can reduce your cost.

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🧰 Professional Installation vs. DIY

While some people tackle their own through-the-wall AC installation, I always recommend using a pro if you want a seamless fit and optimal performance. Poor installation can cause:

• Air leaks that reduce efficiency

• Improper draining that leads to water damage

• Poor sealing around the wall sleeve that invites pests or water infiltration

Investing in a pro means:

• Proper fitting and sealing

• Safe handling of refrigerant lines and electrical connections

• Correct tilt so condensation drains outside — not into your wall

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✅ Conclusion: Pick the Right Unit for Maximum Comfort and Efficiency

Choosing the correct size for your through-the-wall air conditioner is one of the most important decisions you’ll make. Too small and you won’t stay cool; too big and you’ll waste energy and money. Taking into account your space, ceiling height, insulation, heat load, and energy goals will help you pick a unit that hits the sweet spot.

And remember — BTUs alone don’t tell the whole story. Efficiency matters too. Models with R-32 refrigerant and high EER/SEER ratings cost less to run and help protect the environment.

If you’re ready to shop or want to compare models, visit The Furnace Outlet’s full selection of through-the-wall air conditioners. Our team is also here to help you pick the perfect size and model for your home.

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✍️ About the Author

Tony Marino is a veteran HVAC technician with 20+ years in the trade. Known for his no-nonsense advice and practical solutions, Tony shares his knowledge so homeowners can make smarter decisions — and stay comfortable all summer long.