Understanding kW and BTUs: How to Compare Furnace Power Ratings

👋 Introduction: Tony’s Take on Furnace Numbers

Hey folks — Tony here. Over the years, I’ve seen plenty of homeowners make one common mistake when shopping for a furnace: they compare models based on brand and price but skip the most important numbers — the power ratings.

You’ll see things like:

“20 kW Electric Furnace”
“80,000 BTU Gas Furnace”
“2,000 CFM Airflow”

And it all starts to sound like alphabet soup. But those numbers aren’t random — they describe exactly how much heat your system can produce, how fast it can deliver it, and whether it’s the right match for your home.

So today, I’m breaking it down. I’ll explain what kilowatts (kW) and British Thermal Units (BTUs) mean, how they connect, and how you can use them to confidently compare electric and gas furnaces — just like a pro.

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⚙️ 1. What BTUs and kW Really Mean

Both kW and BTUs measure heating power, just in different languages — one is electrical, the other thermal.

🔥 BTU (British Thermal Unit): The Old Reliable

A BTU is the amount of heat needed to raise one pound of water by one degree Fahrenheit.

In HVAC terms, we use BTUs per hour (BTU/h) to measure how much heat a furnace can deliver continuously.

Most gas and oil furnaces are rated in BTU/h — you’ll see models listed as:

• 60,000 BTU

• 80,000 BTU

• 100,000 BTU

So when you read “80,000 BTU furnace,” it means it can deliver roughly 80,000 BTU of heat energy to your home every hour — assuming full output and ideal conditions.

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⚡ kW (Kilowatt): The Modern Electric Standard

Now, 1 kilowatt (kW) equals 1,000 watts, or simply put, a measure of electrical power.

Electric furnaces don’t burn fuel — they use electricity to heat metal coils, which warm the air moving through the ductwork. Because all electrical energy converts directly into heat, electric furnaces are 100 % efficient at point of use.

But here’s the catch: the electric industry measures output in kilowatts, while the heating industry traditionally uses BTUs.

To compare apples to apples, we use this conversion:

🔁 1 kW = 3,412 BTU/hour

So a 20 kW electric furnace produces:

20 × 3,412 = 68,240 BTU/hour

That’s roughly the same heating output as an 80,000 BTU gas furnace running at 85 % efficiency.

(Source: Energy.gov – Home Heating Systems)

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🧾 2. How to Compare Furnace Power Ratings

When you’re switching from gas to electric, or simply trying to match heating capacity, the easiest way to start is by converting both ratings into the same unit.

This means that if your home currently uses a 70,000–80,000 BTU gas furnace, then a 20 kW electric furnace should provide similar comfort levels — as long as your insulation, ductwork, and climate are comparable.

(Reference: ACDirect Furnace Sizing Calculator)

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🧮 3. Real Example: Tony’s Goodman 20 kW Furnace

Let’s look at the exact unit I installed last year:

• Model: Goodman MBVK20DP1X00 Electric Furnace + HKTAD201 Kit

• Output: 20 kW = 68,240 BTU/hour

• Airflow: 2,000 CFM

• Voltage: 240 V, single phase

• Breaker: Two × 60 A circuits

That furnace replaced an older 80,000 BTU gas unit that had seen better days. The new electric system runs quieter, doesn’t need venting, and evenly heats my 2,200 sq ft home.

Because 1 kW = 3,412 BTU, I can confidently say the heating power between my old and new systems is almost identical — except now, every bit of energy I pay for becomes heat.

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🧊 4. Why the Same BTU Doesn’t Always Feel the Same

Let me tell you something I’ve seen on hundreds of job sites: Two furnaces can have identical BTU ratings and still feel completely different inside the home.

Here’s why:

• Gas furnaces lose 5–15 % of heat through the exhaust vent — even high-efficiency ones.

• Electric furnaces transfer nearly all energy into airflow, making heat delivery steadier.

• Duct design and airflow affect comfort as much as the BTU rating.

• Thermostat cycling plays a huge role in maintaining even temperatures.

For example, an 80,000 BTU gas furnace might provide 72,000 BTU of usable heat (90 % AFUE). A 20 kW electric furnace delivers 68,240 BTU — but every single one of those BTUs goes into the air you feel.

That’s why electric heat tends to feel more consistent, even at slightly lower nominal ratings.

(Source: Energy Vanguard – Airflow & Duct Efficiency)

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🌡️ 5. Factoring in Climate & Insulation

The furnace you choose doesn’t just depend on power — it depends on where you live and how well your home holds heat.

If your home is in a cold region like Michigan or Maine, that same 20 kW unit will heat a smaller footprint than it would in Tennessee or Texas.

Also, insulation quality and window efficiency make a huge difference. A well-sealed home in Minnesota might need less heat than a drafty 1950s ranch in Georgia.

(Reference: PickHVAC – Furnace Sizing by Climate)

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⚡ 6. Input vs. Output: The Efficiency Equation

Many homeowners misunderstand “input” and “output” ratings. Here’s the quick rundown:

• Input rating: The total energy consumed (gas or electricity).

• Output rating: The usable heat delivered to your ducts.

Gas furnaces might list both numbers on the data plate. Electric furnaces? The two are the same because there’s no combustion loss.

That’s what makes electric so simple — you get what you pay for, literally.

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💡 7. Why Efficiency Ratings Matter

Understanding efficiency helps you plan both your energy costs and environmental footprint.

• Gas Furnaces: Rated by AFUE (Annual Fuel Utilization Efficiency) — most modern ones fall between 80 % – 96 %.

• Electric Furnaces: Always 100 % efficient at the point of use.

• Heat Pumps: Measure efficiency using COP (Coefficient of Performance) — typically 2 – 4 in mild weather, meaning 200–400 % efficiency.

So while electric resistance heating is efficient, combining it with a heat pump for primary heating gives you the best of both worlds — efficiency and reliability.

(Source: EnergyStar – Furnace Efficiency Standards)

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⚖️ 8. Real Cost Breakdown: kW vs. BTU in Dollars

Let’s put some real math behind this.

At face value, gas looks cheaper. But here’s what most charts ignore:

• Electric furnaces need no venting, saving on installation.

• They require zero combustion maintenance.

• You can pair them with solar panels or off-peak utility plans.

When I factored in rebates and solar generation, my effective cost dropped below $0.09 / kWh — suddenly, my electric heat was on par with gas.

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🧰 9. Tony’s Tips for Reading Power Ratings Like a Pro

After installing and comparing hundreds of systems, here are my top lessons:

✅ Tip 1: Always Convert to the Same Unit

Use 1 kW = 3,412 BTU/h.
If you’re upgrading or comparing gas vs. electric, you can’t make sense of the numbers otherwise.

✅ Tip 2: Check the Label for Output, Not Input

Don’t be fooled by big “INPUT” numbers on gas furnaces — what matters is usable output.

✅ Tip 3: Account for Climate Zone

The colder your winters, the more BTUs you’ll need per square foot.

✅ Tip 4: Don’t Oversize “Just in Case”

A furnace that’s too big will short-cycle, create uneven heating, and waste energy.

✅ Tip 5: Mind Your Electrical Panel

A 20 kW electric furnace draws about 83 amps at 240 V — make sure you have a 200 A service panel to handle it safely.

✅ Tip 6: Match Airflow (CFM)

If your ducts can’t move enough air, even the best-sized furnace will underperform.

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🧭 10. Wrapping Up: Decoding Furnace Power Like a Pro

So what’s the takeaway?

• BTUs measure heat output in thermal terms.

• kW measures electrical energy used to create that heat.

• The conversion — 1 kW = 3,412 BTU/h — lets you directly compare systems.

• Efficiency, climate, and duct design all affect how those numbers translate to comfort.

For my home, switching from a gas furnace rated at 80,000 BTU to a 20 kW electric Goodman system delivered equal performance, quieter operation, and lower maintenance — not to mention cleaner air and fewer emissions.

When you understand the math behind the metal, you can finally pick your next furnace with confidence — and maybe even brag to your HVAC guy about knowing your BTUs from your kWs.

In the next topic we will know more about: What Is CFM Airflow and Why It Matters for Electric Furnaces