Watts to BTU Per Hour Converter

The Watts to BTU per Hour converter translates any electrical or thermal power value between the SI watt (W) and the British thermal unit per hour (BTU/h). Enter a value in either field and the other updates live, so you can move freely between the two conventions. The converter also displays the result in kilowatts (kW) and tons of refrigeration (TR) because those are the units you will see on almost every air-conditioner, furnace, and heat-pump specification sheet in North America.

The conversion is useful for sizing air conditioners against the electrical load they place on a circuit, comparing European (W) and US (BTU/h) equipment catalogs side by side, choosing a portable space heater that will not trip a 15 A breaker, reading historical appliance manuals that quote BTU/h but not watts, working through thermodynamics homework that mixes both units, and sizing generators or solar PV systems against thermal loads expressed in BTU/h.

How to Use the Watts to BTU Per Hour Converter

1. Type a number into Watts (W). The BTU per hour (BTU/h) field updates as you type.
2. Or type into BTU/h; the watts field updates in the opposite direction.
3. Use the ⇄ Swap button between the two fields to flip which value sits in which box.
4. Read the Current value line below the inputs for the canonical "X W = Y BTU/h" reading.
5. The kW and tons of refrigeration (TR) cards below the main result give the same quantity in two other units common on HVAC nameplates and breaker labels.
6. Click Copy on any card, or Copy full result at the bottom, to paste the value into an email, spreadsheet, or load calculation.
7. Use the quick presets (hair dryer, space heater, 1 ton AC, whole-home heater) to load a real-world W ↔ BTU/h pair with one click.

The Conversion Formula

The conversion between watts and BTU per hour uses a single exact constant defined by international standards bodies:

Watts to BTU per hour: BTU/h = W × 3.412141633

BTU per hour to watts: W = BTU/h × 0.29307107

The factor 3.412141633 is the reciprocal of 0.29307107 (because 3.412141633 × 0.29307107 = 1.0000000000 exactly when both are quoted to nine decimals). It is derived from two definitions:

• The International Table (IT) British thermal unit is defined as exactly 1055.05585262 joules.
• One hour is exactly 3600 seconds, and one watt is exactly one joule per second.

Therefore:

• 1 BTU/h = 1055.05585262 J / 3600 s = 0.29307107017222… W (rounded to eight decimals as 0.29307107).
• 1 W = 1 / 0.29307107 = 3.412141633 BTU/h.

The same factor is published in NIST Special Publication 811 (the Guide for the Use of the SI Units) and is used throughout the ASHRAE Handbook of Fundamentals for HVAC rating work. There is no measurement uncertainty in the factor itself; the BTU is defined to yield a whole-number J value, and the hour and second are exact.

Why you also see kW and tons of refrigeration

A few practical conversions fall out of the formula for free:

• 1 kilowatt (kW) = 1000 W = 1000 × 3.412141633 = 3412.141633 BTU/h.
• 1 ton of refrigeration (1 TR) = 12,000 BTU/h by definition (US "short ton" cooling ton). In watts: 12,000 × 0.29307107 = 3516.85284 W, which is 3.516853 kW.
• A 2-ton AC is therefore 24,000 BTU/h ≈ 7,033.71 W ≈ 7.03 kW.
• A 3-ton AC is therefore 36,000 BTU/h ≈ 10,551.0 W ≈ 10.55 kW.

The "ton" unit is a historical fossil: it represents the heat a one-ton block of ice absorbs melting over 24 hours, which by coincidence equals 12,000 BTU. Today "tons of AC" is just shorthand for "12,000 BTU/h of cooling capacity per ton."

Quick Reference Table

Heating and cooling capacity numbers are power. A 1500 W space heater that runs for one hour delivers 1500 Wh = 1.5 kWh of electrical energy, which (assuming 100% conversion inside the room) becomes roughly 5118 BTU of heat energy. The reverse is also true: a 12,000 BTU/h air conditioner that runs for one hour removes about 12,000 BTU (≈ 3.5 kWh of heat) from a room, the electrical input is lower than 3517 W because of efficiency (the AC moves heat rather than creating cold).

Worked Examples

Example 1, A 1,000 W portable appliance

A European 230 V kettle is rated 1,000 W. What is that in BTU/h for a US spec sheet?

BTU/h = 1000 × 3.412141633 = 3412.1416 BTU/h.

So the kettle delivers about 3412 BTU per hour of heat into the room when it runs. A 1000 W kettle and a 1000 W fan heater produce essentially the same heat output.

Example 2, A 1 ton (12,000 BTU/h) AC unit

A bedroom window / mini-split air conditioner is rated 12,000 BTU/h of cooling. What is the equivalent electrical input in W and kW?

W = 12,000 × 0.29307107 = 3516.85 W ≈ 3.52 kW.

On a 120 V North American circuit, that is about 29.3 A of running current, too much for a 15 A wall outlet, which is why most 12,000 BTU/h units ship with a 240 V plug (or a 20 A / 30 A 120 V plug, depending on model). On a 230 V European circuit it is only about 15.3 A, well within a standard 16 A outlet.

Example 3, A 5,000 W whole-home electric heater

A whole-home electric furnace is rated 5,000 W (5 kW). What is that in BTU/h?

BTU/h = 5000 × 3.412141633 = 17,060.7 BTU/h.

That is roughly equivalent to a 1.4-ton AC's worth of heating capacity. A typical 2000 sq ft home in a moderate climate needs around 50,000 to 80,000 BTU/h on the coldest design day, so a 5 kW electric strip would not be sufficient on its own, a heat pump or gas furnace would be needed.

Example 4, Converting a generator rating

A portable generator is rated 3000 W continuous. The owner's manual claims it can run a "13,500 BTU/h RV air conditioner." Check: 13,500 × 0.29307107 = 3956.5 W, which is more than the 3000 W continuous rating, the AC would trip the generator on startup surge, even though it might run on a 4000 W peak rating.

Where This Conversion Shows Up in Practice

• Air-conditioner shopping. US spec sheets list cooling capacity in BTU/h; European and most modern US inverter units list it in W or kW. Converting lets you compare catalogs.
• Generator and inverter sizing. A 5-ton (60,000 BTU/h) AC requires about 17.6 kW of electrical input at full load, plus headroom for compressor startup surge.
• Solar PV design. A roof's heating or cooling load in BTU/h translates into a kWh-per-day figure once you convert to watts and apply hours-of-use.
• Electrician load calculations. A US 120 V / 15 A outlet can deliver at most ~1800 W continuously; a 1500 W space heater is the practical ceiling. A 12,000 BTU/h AC at ~3517 W is too much.
• Thermodynamics homework. Many textbooks still quote heating values in BTU and BTU/h, while modern problems expect SI units.
• International appliance shopping. Converting lets you check that a heater bought in Europe will draw the right current on a US circuit (or vice versa).

Common Mistakes to Avoid

1. Confusing cooling capacity with electrical input. A "12,000 BTU/h" AC has a cooling effect of 12,000 BTU/h, but its electrical input is only about 1000 to 1500 W because it is moving heat rather than generating it. The conversion factor itself is identical for both, but the numbers are very different because of the air conditioner's coefficient of performance (typically 2.5 to 4.0).
2. Forgetting the "per hour". BTU (without the /h) is a unit of energy, not power. 1 BTU ≈ 1055 J. BTU/h is power. A 12,000 BTU/h AC running for one hour removes 12,000 BTU of heat; the per hour is what makes the conversion to watts exact.
3. Using the wrong ton. The "ton" in HVAC almost always means 12,000 BTU/h (US short ton cooling ton). The "metric tonne cooling ton" sometimes used in European literature is 50,000 / 50 ≈ the same order but a slightly different convention. This converter uses the US short ton (1 TR = 12,000 BTU/h = 3516.85 W) because that is what US, Canadian, Japanese, and most global air-conditioner nameplates use.
4. Rounding too aggressively. Using "1 W = 3.4 BTU/h" introduces about a 0.36% error, small but noticeable on a 36,000 BTU/h (3-ton) AC where it costs you 130 BTU/h of accuracy. The exact 3.412141633 / 0.29307107 factors are precise enough that you do not lose significant digits until the seventh.
5. Confusing BTU/h with BTU/(h·°F) or similar flow units. Heat flow rates (U-value, UA) sometimes use BTU per hour per degree Fahrenheit; those are different units entirely and do not convert directly to watts. This converter is only for plain BTU/h (a power unit).

Frequently Asked Questions

Q: What is the exact formula to convert watts to BTU per hour? A: Multiply the watt value by 3.412141633. Example: 1000 W × 3.412141633 = 3412.1416 BTU/h. The factor is the reciprocal of the IT BTU definition (1 BTU = 1055.05585262 J exactly) divided by 3600 s/h.

Q: What is the formula to convert BTU per hour to watts? A: Multiply the BTU/h value by 0.29307107. Example: 12,000 BTU/h × 0.29307107 = 3516.85 W. The factor comes from 1055.05585262 J ÷ 3600 s.

Q: How many BTU per hour is 1,000 watts? A: 1000 W = 3412.1416 BTU/h (often rounded to 3412 BTU/h for spec sheets). That is also exactly 1 kW = 3412.14 BTU/h.

Q: How many watts is a 5,000 BTU window AC? A: A 5,000 BTU/h window AC corresponds to 1465.4 W. Note that the cooling capacity is 5000 BTU/h, but the electrical input to a typical 5000 BTU/h window unit is only about 450 to 550 W because the AC moves heat rather than creating it.

Q: What is a "ton" of air conditioning in watts? A: One ton of refrigeration (1 TR) equals exactly 12,000 BTU/h, which is 3516.85 W or 3.5169 kW. A "2-ton" AC is 24,000 BTU/h ≈ 7033.7 W. The term comes from the cooling effect of melting one short ton (2000 lb) of ice per day.

Q: How many watts is a 12,000 BTU/h air conditioner? A: 12,000 BTU/h × 0.29307107 = 3516.85 W ≈ 3.52 kW. That is the cooling capacity in electrical units; the actual plug load of a typical 12,000 BTU/h unit is closer to 1000 to 1500 W because of its efficiency.

Q: How many BTU/h is a 1,500 W space heater? A: 1500 W × 3.412141633 = 5118.21 BTU/h. Electric resistance heat is 100% efficient, so a 1500 W space heater really does deliver about 5118 BTU of heat per hour into the room.

Q: Is 1 W equal to 3.41214 BTU/h or 3.412141633 BTU/h? A: Both are correct to different precisions. 3.41214 is the rounded 6-decimal form; 3.412141633 is rounded to nine decimals. The exact value is 1 / 0.29307107 = 3.4121416331276... For any practical HVAC calculation the 6-decimal form is fine, and the converter uses the 9-decimal form internally so it stays accurate even at large values.

Q: Can I use this converter to size a generator or solar inverter? A: Yes, the underlying conversion is mathematically exact and suitable for engineering estimates. For final sizing of a standby generator, off-grid inverter, or NEC-compliant feeder, also account for startup surge (compressors draw 2 to 4× their running wattage for the first second), power factor, and the relevant safety margin, usually 20 to 25% above the running load.

Q: How accurate is the 0.29307107 / 3.412141633 factor? A: The factor is exact to eight significant digits, derived from the IT BTU definition (1055.05585262 J) and the SI definition of the hour (3600 s). The full unrounded factor is 0.29307107017222... W per BTU/h. For any nameplate rating, even large commercial chillers and boilers, rounding to seven significant digits is well below the precision of the equipment itself.

References

• NIST Special Publication 811: Guide for the Use of the SI Units, defines the SI power unit (watt) and gives the conversion factor between BTU/h and watts.
• ASHRAE Handbook of Fundamentals, canonical HVAC reference; equipment ratings in BTU/h and equivalent SI watt values are listed in the heating and cooling load chapters.
• BIPM SI Brochure (9th edition, 2019), formal definition of the watt and the relationship between SI and non-SI energy and power units.
• International Table British thermal unit (IT BTU), ISO 31-4 and the original 1929 International Steam Tables Conference, where the IT BTU was fixed at 1055.05585262 J exactly.
• ANSI/AHRI Standard 210/240, performance rating of unitary air-conditioning and air-source heat pump equipment, which uses BTU/h as the primary capacity unit.
• ANSI/AHRI Standard 220, thermal storage equipment ratings, which use BTU/h for capacity and equivalent kW values.

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