Heating equipment sold in Europe and much of the world is rated in kilowatts (kW). In the United States, heating capacity is typically listed in BTUs per hour. If you're comparing a European-made boiler to a US-spec furnace, shopping for a mini-split with specs in mixed units, or trying to understand a heating contractor's quote, you need to speak both languages. The conversion is simple — and knowing it prevents costly sizing mistakes.
One kilowatt equals exactly 3,412 BTU per hour. That's a fixed physical constant, not an approximation.
The number 3,412 comes from the definition of energy units: one watt is one joule per second, and one BTU is approximately 1,055 joules. Work through the unit conversions and you land at 3,412 BTU per kilowatt-hour.
One important distinction: kW measures a rate of energy output (power), and so does BTU/hr. You're not converting energy stored — you're converting the rate at which a heating system produces heat. Always make sure both sides of your comparison are in per-hour terms before applying the formula.
A homeowner is comparing two heating options for a 1,500 sq ft home:
To compare them directly, convert Option A to BTU/hr:
24 kW × 3,412 = 81,888 BTU/hr
The two systems are nearly identical in heating capacity — within about 2% of each other. The combi boiler at 24 kW and the furnace at 80,000 BTU/hr will heat roughly the same sized space under similar conditions.
Now suppose the homeowner sees a third option: a heat pump rated at 7 kW of heating output:
7 × 3,412 = 23,884 BTU/hr
That's closer to a 2-ton unit — appropriate for a well-insulated 1,000–1,200 sq ft space, not 1,500 sq ft. Without the conversion, it would be easy to assume 7 kW "sounds like enough" without realizing it falls short for the intended space.
1. Confusing input kW with output kW.
A furnace or boiler has an input rating (how much energy it consumes) and an output rating (how much heat it delivers). Efficiency determines the gap. A 30 kW input boiler running at 90% efficiency delivers 27 kW of heat output. Always use the output figure when sizing — using input kW overstates actual heating capacity.
2. Forgetting the "/hr" — and mixing up kWh with kW.
kWh (kilowatt-hours) is a measure of total energy, like how much you see on your electricity bill. kW is a rate of power. When converting to BTU/hr, you need kW (power), not kWh (energy). Using the wrong figure throws off your calculation entirely.
3. Assuming all BTU ratings are the same standard.
Heating equipment BTU ratings can refer to input or output capacity, and some manufacturers list both. Gas furnaces in the US typically advertise input BTU/hr prominently, with output efficiency-adjusted. Heat pumps list heating output. Always read the spec sheet carefully to confirm what the listed BTU figure actually represents.
1. Comparing imported and domestic heating equipment.
If you're sourcing a boiler or heat pump from a European manufacturer — increasingly common in the US as heat pump adoption grows — you'll need to convert kW to BTU/hr to compare it against local quotes and local sizing guidelines.
2. Interpreting mini-split specifications.
Many ductless mini-split systems, even those sold in the US, list both kW and BTU ratings in their spec sheets. Cross-checking the two figures using the formula is a quick way to catch typos or mismatched spec sheets before you buy.
3. Sizing a backup electric heater.
Portable and supplemental electric heaters are often sold in watts or kilowatts. A 1.5 kW space heater delivers 1,500 × 3.412 = 5,118 BTU/hr — useful when trying to understand whether it can realistically take the chill off a room or just provide a small boost.
Is 1 kW always equal to 3,412 BTU/hr?
Yes — it's a fixed conversion based on physical constants. It doesn't vary with efficiency, fuel type, or equipment brand. The only thing that changes is whether you're looking at input or output figures.
How many kW is a typical home furnace?
A standard US residential gas furnace ranges from about 60,000 to 120,000 BTU/hr input, which is roughly 18 to 35 kW. Output after efficiency losses is typically 80–95% of that figure depending on the AFUE rating.
What's the difference between kW and kWh in a heating context?
kW is how fast your system heats (power). kWh is how much total energy it uses over time (consumption). A 10 kW heater running for 3 hours uses 30 kWh of energy — and that's what shows up on your electricity bill.
Can I use this formula for cooling as well?
Yes. Air conditioner capacity is also measured in BTU/hr, and the same conversion applies. A 5 kW cooling output equals approximately 17,060 BTU/hr — just under 1.5 tons of cooling.
The kW to BTU/hr conversion is a single multiplication by 3,412 — but knowing when and how to apply it correctly separates a well-matched heating system from an expensive misfit. Always confirm whether specs are input or output before comparing.