You walk into a hardware store. You look at the window units. They all look roughly the same. White boxes. Grilles. A plug.
But the numbers on the boxes tell a different story.
Most units are rated in British thermal units, or Btu. It’s a measure of heat energy. One Btu raises the temperature of one pound of water by one degree Fahrenheit. In HVAC terms, one ton of cooling power equals 12,000 Btu.
A standard window unit might sit at 10,000 Btu. A whole-house system for a 2,000-square-foot home? That’s about 60,000 Btu. Five tons.
That implies roughly 30 Btu per square foot.
This is a rough estimate. It is not a rule. If you need to size an air conditioner accurately for your specific space, call an HVAC contractor. Do not guess with central systems.
For a single window unit, you have more control.
The EER Explained Simply
The Energy Efficiency Ratio (EER) tells you how much cooling you get for every watt of electricity.
It is a simple division problem.
Take the Btu rating. Divide it by the wattage.
Example: A 10,000-Btu unit drawing 1,200 watts.
10,000 / 1,200 = 8.3 EER.
Higher EER is better. It means less wasted energy. But it costs more upfront.
You are trading money now for money later.
The Payback Period Calculation
Let’s look at two units. Both are 10,000 Btu.
Unit A: EER of 8.3. Uses 1,200 watts. Costs $100 less.
Unit B: EER of 10. Uses 1,000 watts. Costs $100 more.
You want the efficient one (Unit B). But is the extra $100 worth it?
You need two numbers.
- How many hours per year will you run it?
- What is the cost per kilowatt-hour (kWh) in your area?
Assume you run the AC six hours a day for four months. Assume electricity costs $0.10 per kWh.
The difference in power draw is 200 watts. Every five hours, the inefficient unit burns an extra kilowatt-hour. That’s $0.10.
Here is the math.
30 days x 4 months = 120 days.
120 days x 6 hours = 720 hours.
720 hours x 200 watts = 144,000 watt-hours.
144,000 watt-hours / 1,000 = 144 kWh.
144 kWh x $0.10 = $14.40 saved per year.
The expensive unit costs $100 more. You save $14.40 a year.
$100 / $14.40 = roughly 7 years.
Seven years to break even.
If you move in five years, you lose money. If you stay for twenty, you win.
This is how you calculate the payback period on AC efficiency. It depends entirely on your climate and your habits.
The Humidity Factor
Btu and EER handle heat. They do not handle humidity.
Humans cool themselves by sweating. It works best when relative humidity sits around 45 percent.
High humidity changes everything. The air gets saturated. Sweat does not evaporate. It sits on your skin. You feel sticky. Hot. Uncomfortable.
Your body tries to compensate. You feel the heat more intensely. The AC might drop the temperature number, but if it isn’t pulling moisture out of the air, you still feel warm.
“Think of that shiny glow as your body’s personal AC — when it isn’t too humid out.”
This is why a unit with a dehumidification feature matters
