Introduction
Hey, I’m Mike Sanders. If you’re shopping for a through-the-wall air conditioner, you’re probably wondering, “What size do I really need?” Picking the right size isn’t guesswork — it’s about matching the unit’s cooling power to your home’s specific needs.
In this guide, I’ll walk you through everything from BTU ratings and room size calculations to insulation factors and installation considerations. By the end, you’ll know how to pick the right through-the-wall AC that cools efficiently and saves you money.
🔥 What Does BTU Mean and Why It Matters
BTU, or British Thermal Unit, measures how much heat an air conditioner can remove from your room each hour. The higher the BTU, the more cooling power.
Picking a unit that’s too small means it runs constantly, struggling to cool the space, driving up energy bills and wear. Too big, and it cools too fast without removing humidity, making the room feel clammy.
For through-the-wall units, BTUs typically range from 5,000 to 18,000 depending on your room size. If you want to learn more about how BTUs impact your cooling, Energy Star has a great overview on choosing the right air conditioner that breaks down BTU sizing.
📏 Calculating the Right Size: Room Square Footage
Start by measuring your room: length × width = square footage.
| Square Footage | Recommended BTU Capacity |
|---|---|
| Up to 150 sq ft | 5,000 BTUs |
| 150 - 250 sq ft | 6,000 BTUs |
| 250 - 350 sq ft | 8,000 BTUs |
| 350 - 450 sq ft | 10,000 BTUs |
| 450 - 550 sq ft | 12,000 BTUs |
| 550 - 700 sq ft | 14,000 BTUs |
| 700 - 1,000 sq ft | 18,000 BTUs |
If you want a step-by-step walkthrough on measuring your space and choosing the right size AC, Family Handyman has an excellent how-to guide on measuring for window and wall air conditioners.
🏠 Consider Your Room’s Insulation and Sun Exposure
Insulation and sunlight make a big difference. Rooms that aren’t well insulated or get a lot of direct sun require higher BTU ratings to compensate for heat gain. Conversely, shaded or well-insulated rooms might need a slightly smaller unit.
Here’s a quick rule of thumb: add 10-20% more BTUs for sunny rooms, add 4,000 BTUs if you’re cooling a kitchen, and subtract 10% for shaded rooms.
For a deeper dive into energy efficiency and insulation impact, the EPA offers a useful energy-efficient cooling guide that’s worth checking out.
🌬️ Airflow and Ventilation Impact
Good airflow and sealed windows/doors reduce how hard your AC has to work. If your home leaks air or lacks ventilation, your AC will run longer to maintain comfort.
If you haven’t already, it’s a good idea to check for drafts and seal leaks before investing in a new AC. It’ll improve comfort and save energy.
🔧 Wall Sleeve Size and Installation Constraints
Practical tip: your wall sleeve’s size limits what unit you can install. Not every BTU rating fits all wall openings.
Make sure to measure your existing wall sleeve or opening and compare it to the dimensions of your through-the-wall AC. This saves you from ordering a unit that won’t fit.
💡 Energy Efficiency Ratings Matter
Look for units with a high Energy Efficiency Ratio (EER) or Seasonal Energy Efficiency Ratio (SEER). Higher ratings mean lower electricity bills and better performance.
Many newer models on The Furnace Outlet use R-32 refrigerant, which is more environmentally friendly and efficient compared to older refrigerants like R-410A.
For more on the benefits of R-32 and why it’s becoming the industry standard, check out this detailed refrigerant guide.
🛠️ Bonus Tips for Tricky Spaces
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If your room is oddly shaped, break it into zones and add up the BTUs needed for each.
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High ceilings? Add about 10% more BTUs.
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Rooms with lots of electronics or heat-generating appliances? Increase capacity accordingly.
Wrapping Up: Your Perfect Fit
Sizing your through-the-wall air conditioner right means balanced comfort and efficiency — no overheating, no wasted energy. It’s all about knowing your space, factoring in insulation, sunlight, and ventilation.
When you’re ready, explore the full selection of units tailored to various room sizes and budgets at The Furnace Outlet’s through-the-wall AC collection.
If you want to learn more about installation tips, maintenance, or which models come with smart features, stay tuned for the next posts in this series.
