BTU to Watts Converter

The BTU to Watts converter translates any power value between British thermal units per hour (BTU/h) and the SI watt (W). For the BTU to Watts Converter, Enter a value in either field and the other updates instantly. The converter is useful for HVAC technicians sizing air conditioners, electricians sizing backup generators and inverter loads, homeowners comparing appliance ratings between US (BTU/h) and European (W) catalogs, energy auditors quoting heating and cooling loads, solar installers sizing PV arrays to cover thermal loads, and students working through thermodynamics problems that mix the two conventions. The conversion is exact, the single factor (1 BTU/h = 0.29307107 W) is defined by international standards bodies (NIST, BIPM) and is used throughout the ASHRAE Handbook of Fundamentals for HVAC rating work.

How to Use the BTU to Watts Converter

1. Type a number into the BTU per hour (BTU/h) field to see the watt equivalent update on the right.
2. Type a number into the Watts (W) field to see the BTU/h equivalent update on the left.
3. For the BTU to Watts Converter, The two fields stay in sync as you type, editing one updates the other automatically.
4. Use the quick presets for typical HVAC equipment (window AC, central AC, space heater, heat pump, whole-home heater) to load an exact BTU/h ↔ W pair instantly.
5. Read the result box for the active formula, the conversion factor, and a side-by-side display of the current value in both units.
6. The reverse-conversion panel below the main box re-states the W → BTU/h value explicitly so you can confirm the round trip.

The Conversion Formula

The conversion between BTU per hour and watts uses a single exact constant: the factor 0.29307107 watts per BTU/h. Formally:

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

Watts to BTU/h: BTU/h = W ÷ 0.29307107

Where does 0.29307107 come from? The IT (International Table) British thermal unit is defined as exactly 1055.05585262 joules. One watt is one joule per second, so one hour is 3600 seconds, and:

1 BTU/h = 1055.05585262 J / 3600 s = 0.293071070172222… W

The converter rounds to seven significant digits (0.29307107), which is more than enough precision for every practical HVAC rating and matches the factor published in NIST SP 811 and used by the ASHRAE Handbook. Reciprocally, 1 W = 1 / 0.29307107 = 3.4121416 BTU/h.

Note on furnace inputs vs outputs. A natural-gas furnace rated "80,000 BTU/h input" delivers about 72,000 BTU/h of useful heat output after combustion losses (typical 90% AFUE). The conversion factor is identical for input and output, both are power. The discrepancy is between what is burned and what heats the house, not a unit issue.

Worked Examples

Example 1, A 12,000 BTU/h window air conditioner. A bedroom window unit is rated 12,000 BTU/h. What is that in watts? W = 12,000 × 0.29307107 = 3,516.85 W. So 12,000 BTU/h ≈ 3.5 kW. On a 120 V US circuit, that is about 29.3 A of running current, too much for a standard 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).

Example 2, A 1,500 W electric space heater. A typical US portable space heater is rated 1,500 W. What is that in BTU/h? BTU/h = 1,500 ÷ 0.29307107 = 5,118.21 BTU/h. So 1,500 W ≈ 5,118 BTU/h of heat delivered into the room. The figure is useful for sizing a room: a 150 sq ft bedroom needs roughly 4,000 to 5,000 BTU/h to stay warm in a moderate winter, so a single 1,500 W heater is the right order of magnitude.

Example 3, A 24,000 BTU/h (2-ton) central AC. A 2-ton central air conditioner is rated 24,000 BTU/h. What is that in watts? W = 24,000 × 0.29307107 = 7,033.71 W. So 24,000 BTU/h ≈ 7.03 kW of cooling input. Adding the blower and outdoor fan (typically another 500 to 800 W combined), the total electrical load is roughly 7.5 to 7.8 kW. A dedicated 30 A or 40 A 240 V circuit handles this comfortably; a backup generator covering the AC alone needs at least 8 kW of capacity.

Example 4, Sizing a backup generator for a 3-ton heat pump. A 3-ton (36,000 BTU/h) heat pump in cooling mode draws roughly: W = 36,000 × 0.29307107 = 10,550.6 W. Plus the indoor air handler (~600 W) and outdoor fan, the total is around 11.5 kW. A backup generator for this load must be rated at least 14 to 16 kW to cover the locked-rotor startup surge (which can be 3 to 5× the running watts for a few seconds). This is why whole-home generator sizing almost always starts from the watt figure rather than the BTU/h figure on the data plate.

Example 5, Round trip. Verify that converting BTU/h → W and back returns the original value. Start: 8,500 BTU/h. W = 8,500 × 0.29307107 = 2,491.10 W. BTU/h = 2,491.10 ÷ 0.29307107 = 8,500.0 BTU/h. (Round-off at 7+ digits is invisible in any real rating.) This round trip is a useful sanity check when comparing two converters or auditing a calculation.

Where the Conversion Shows Up

• HVAC equipment ratings. Window AC, mini-split, central AC, and heat pump units in North America are routinely rated in BTU/h; European and Japanese equivalents are quoted in watts (or sometimes kilowatts). A bilingual spec sheet often carries both numbers for the same unit.
• Generator and inverter sizing. Whole-home backup generators, portable inverter generators, and battery-based inverter/charger systems are all rated in watts (or kilowatts). Converting the HVAC load to watts lets the generator rating be compared directly with the appliance nameplate.
• Solar PV design. When sizing a PV array to offset a known thermal load (e.g. "we run a 24,000 BTU/h AC for 8 hours a day in summer"), the load first becomes kWh, then kW of PV, then panels, every step downstream is in watts.
• Heater and boiler labels. Electric resistance heaters and electric boilers are usually rated in watts or kilowatts. Gas furnaces and boilers are rated in BTU/h. Spec sheets that list both side by side let you compare across fuel types.
• Energy audits and retrofit work. An energy auditor quoting the heating load of a house often carries both numbers: BTU/h for legacy US work, watts (or kW) for international reporting and EU energy-labelling compliance.
• Insulation and heat-loss calculations. Heat loss through walls, roofs, and windows is computed in BTU/h per square foot per degree (US) or W/m²/K (SI). The conversion ties the two conventions together so a heat-load calculation can mix inputs from both systems.
• Engineering and physics. Thermodynamics textbooks and the SI definition of the watt both anchor on the joule; the BTU is the practical engineering unit that bridges imperial and SI power. Converting BTU/h to watts is the cleanest way to plug an imperial figure into a watt-based calculation.

Common Mistakes

• Confusing BTU (energy) with BTU/h (power). A BTU is a unit of energy (about 1,055 joules). A BTU/h is a unit of power (one BTU per hour, or about 0.293 W). A "10,000 BTU" window AC actually moves 10,000 BTU/h of heat, the "h" is implied but usually omitted on retail packaging. Treating 10,000 BTU as 10,000 joules (or 10,000 watts) is off by a factor of about 10 million.
• Confusing input vs output power. A furnace rated 80,000 BTU/h input delivers only about 72,000 BTU/h output (at 90% AFUE). A heat pump rated 36,000 BTU/h cooling may use only ~10,500 W of electrical input (because the heat pump moves heat rather than creating it, with a COP of 3 or more). Always check the data plate for which direction the figure refers to.
• Using BTU/h in a kWh calculation without the "per hour." Energy is power × time. To turn a BTU/h rating into kWh per day, you need to know how many hours the appliance runs. A 24,000 BTU/h AC running 8 hours a day delivers 24,000 × 8 = 192,000 BTU/day ≈ 56 kWh/day of thermal load (the electrical input is lower for an AC because it just moves heat).
• Forgetting the factor direction. BTU/h × 0.29307107 gives watts. Watts ÷ 0.29307107 gives BTU/h. A common slip is to divide when multiplying is required (or vice versa), which produces a number about 85,000× too large or too small.
• Rounding too early. A 12,000 BTU/h AC rounded to 12,000 ÷ 3.4 = 3,529 W is fine for casual use, but for electrical design use the full 3,516.85 W. At 120 V that is 29.31 A, not 29.41 A, a real difference when sizing a 30 A breaker that allows only 24 A continuous load.
• Mixing cooling BTU/h and heating BTU/h on the same data plate. Many heat pumps list separate cooling and heating capacities (e.g. "36,000 BTU/h cooling / 40,000 BTU/h heating"), and reversing them gives a wildly wrong result. Always note which number is which on the spec sheet.

Quick Mental Conversion Trick

For an approximate BTU/h → W in your head: divide by 3.4. A 12,000 BTU/h AC → 12,000 / 3.4 ≈ 3,529 W (actual 3,516.85 W). A 24,000 BTU/h central AC → 24,000 / 3.4 ≈ 7,059 W (actual 7,033.71 W). The trick is accurate to within about 0.3%.

For an approximate W → BTU/h in your head: multiply by 3.4. A 1,500 W heater → 1,500 × 3.4 = 5,100 BTU/h (actual 5,118 BTU/h). A 6,000 W whole-home heater → 6,000 × 3.4 = 20,400 BTU/h (actual 20,473 BTU/h).

A more accurate mental form is divide/multiply by 3.412, the reciprocal of 0.29307107. A 12,000 BTU/h AC × 1 / 3.412 ≈ 3,517 W, almost exact. For everyday use the "3.4" trick is close enough; for generator or breaker sizing use the calculator.

When to Use a Different Unit

Watts (W) and kilowatts (kW) are the SI standard for power and are used in every electrical context, generator sizing, PV design, electrical code calculations, and most non-US equipment ratings. BTU/h is still standard for HVAC equipment in the United States and Canada and is also the conventional unit for heating and cooling load calculations in legacy US engineering practice. "Tons of refrigeration" (TR) is another common HVAC unit, where 1 TR = 12,000 BTU/h = 3,516.85 W, a 2-ton AC is exactly 24,000 BTU/h. For very large thermal loads, MBH (thousands of BTU per hour) is common: 1 MBH = 1,000 BTU/h. For absolute-power contexts (gas turbine output, large electric motors), horsepower (hp) and BTU/h coexist: 1 hp ≈ 2,544 BTU/h ≈ 745.7 W.

If you need to convert between BTU/h and tons of refrigeration, divide BTU/h by 12,000 (or watts by 3,516.85). If you need to convert between BTU/h and horsepower, multiply hp by 2,544.22.

Frequently Asked Questions

Q: What is the formula to convert BTU per hour to watts? A: Use W = BTU/h × 0.29307107. Multiply the BTU/h value by 0.29307107. For example, 12,000 BTU/h × 0.29307107 = 3,516.85 W. The factor 0.29307107 comes from the IT (International Table) definition of the BTU combined with the SI definition of the watt: 1 BTU = 1,055.05585262 J exactly, divided by 3,600 s/h.

Q: What is the formula to convert watts to BTU per hour? A: Use BTU/h = W ÷ 0.29307107. Divide the watt value by 0.29307107. For example, 1,500 W ÷ 0.29307107 = 5,118.21 BTU/h. The reciprocal of 0.29307107 is 3.4121416, so multiplying by 3.4121416 gives the same result.

Q: How many watts is a 5,000 BTU window AC? A: A 5,000 BTU/h window AC draws roughly 1,465 W (about 12.2 A on a 120 V circuit). The cooling capacity is 5,000 BTU/h, but the electrical input is somewhat lower because the AC just moves heat rather than creating it, typical EER ratings of 9 to 10 mean the electrical input is closer to 500 to 550 W for a 5,000 BTU/h cooling output.

Q: How many watts is a ton of cooling? A: One ton of refrigeration (1 TR) is defined as exactly 12,000 BTU/h, which is 3,516.85 W. A "2-ton" central AC is therefore 24,000 BTU/h ≈ 7,033.71 W of cooling capacity. The term "ton" originally comes from the cooling effect of melting one short ton (2,000 lb) of ice per day, which is also 12,000 BTU/h.

Q: How do I convert BTU/h to kW? A: Divide watts by 1,000 (or multiply BTU/h by 0.00029307107). A 24,000 BTU/h central AC is 7,033.71 W ÷ 1,000 = 7.03 kW. A 36,000 BTU/h AC is 36,000 × 0.00029307107 = 10.55 kW. The kW form is the usual unit for generator sizing, solar PV design, and breaker/feeder calculations.

Q: How accurate is the 0.29307107 factor? A: The factor is rounded to eight significant digits. The full unrounded value is 0.293071070172222... W per BTU/h, derived from 1 BTU = 1055.05585262 J exactly and 1 hour = 3600 s exactly. For any practical HVAC rating, even the largest chillers and boilers, rounding to seven digits (0.29307107) is well below the precision of the nameplate.

q: can the BTU to Watts Converter be used for professional or commercial purposes? A: Yes, the BTU to Watts Converter provides mathematically correct results that are suitable for professional, commercial, and educational use. For high-stakes applications (medical, legal, financial, code-compliance electrical design), verify results with a domain expert and use the appropriate National Electrical Code (NEC) or ASHRAE load calculation methodology alongside the unit conversion. The conversion factor used is exact by international definition.

For the BTU to Watts Converter, How often are the BTU to Watts Converter formulas updated? A: The formulas are based on the IT (International Table) definition of the British thermal unit and the SI definition of the watt, both of which are fixed by international standards bodies (NIST, BIPM). The 0.29307107 factor has been stable for decades and is not expected to change. Modern precision refinements relate to how the units are realised in laboratory settings, not to the everyday conversion formula.

References

• NIST Special Publication 811: Guide for the Use of the SI Units, defines the SI power unit (watt) and the conversion factors between SI and customary units, including BTU/h.
• ASHRAE Handbook of Fundamentals (most recent edition), the canonical HVAC reference for heating/cooling load calculations, equipment ratings in BTU/h, and equivalent SI watt values.
• BIPM SI Brochure (9th edition, 2019), the 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 (Quantities and units, Part 4: Heat) 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 cooling/heating capacity unit and reports corresponding watts.