Watts to BTU Converter — Free Online Calculator
Convert watts to BTU per hour and vice versa. Essential for HVAC and heating calculations.
How to Use This Calculator
Enter a value and select conversion direction.
The Formula Explained
1 Watt = 3.41214 BTU/hr. 1 BTU/hr = 0.29307 Watts.
Why the Watts-to-BTU Conversion Matters
The watts-to-BTU/hr conversion sits at the intersection of electrical engineering and HVAC design. Electricity is sold and delivered in watts (or kilowatts), but heat and cooling loads are specified in BTU/hr across North America. Every space heater, electric water heater, heat tape circuit, and server room needs both sides of this conversion: how many BTU of heat does 1,500 watts produce, and how many watts does a 24,000 BTU/hr air conditioner draw? The exact conversion factor is 1 watt = 3.412 BTU/hr, and inversely 1 BTU/hr = 0.293 watts.
This matters because electrical loads that convert directly to heat (resistive heaters, incandescent bulbs, electric ovens) are nearly 100% efficient at turning watts into heat. A 1,500-watt space heater produces 5,118 BTU/hr of heat, period. Motors, transformers, and electronics convert most input power to useful work and only produce heat from losses — a 1,000-watt motor might only produce 100-200 BTU/hr of waste heat. Knowing which scenario applies determines whether you size a room for 100% conversion or for loss-only conversion.
Worked Example: Sizing a Server Room Air Conditioner
A small server room has 8 rack units averaging 400 watts each (3,200 watts total), plus 200 watts of network gear and lighting. Total electrical load: 3,400 watts. For servers, essentially 100% of input power becomes waste heat that must be removed by the AC — there is no mechanical work output.
Heat load: 3,400 W × 3.412 = 11,601 BTU/hr. Round up for safety margin and add about 20% for door openings, wall conduction, and occupancy: approximately 14,000 BTU/hr. Specify a 15,000 BTU/hr (1.25 ton) mini-split or dedicated server-room AC. If you undersize based on the raw 11,601 BTU/hr, the unit will run continuously in summer and eventually fail from lack of cycling rest.
Worked Example: Electric Heater Room Sizing
A detached workshop in a cold climate, 400 square feet, needs supplemental electric heating. Rule of thumb: 10 watts per square foot for well-insulated space, 15 W/sq ft for average insulation. For 400 sq ft moderately insulated: 400 × 12 = 4,800 watts needed. That is 4,800 × 3.412 = 16,378 BTU/hr of heat output.
Electrical implications: 4,800 watts at 240V single-phase is 20 amps. A 25-amp breaker with 10 AWG copper wire handles this comfortably. At 120V it would be 40 amps, needing a 50-amp circuit and 8 AWG wire — impractical. Always use 240V for electric resistance heating above 1,500 watts because the wire and breaker savings are dramatic. This is why baseboard heaters and electric water heaters are universally 240V in North America.
Common Conversion Mistakes
1. Confusing BTU and BTU/hr. BTU is energy (like joules); BTU/hr is power (like watts). Appliance ratings are always BTU/hr even when the label abbreviates to just BTU. The watts-to-BTU conversion is really watts-to-BTU/hr.
2. Assuming motors convert all power to heat. A 5 HP motor draws about 3,700 watts but only produces about 300-400 watts of waste heat. The other 3,300 watts becomes mechanical output. Oversize your AC if you assume 100% conversion on motor loads.
3. Forgetting duty cycle. A 4,000-watt oven is only on 30% of the time while cooking. Average heat load is 1,200 watts, not 4,000. Peak vs average matters for room AC sizing.
4. Mixing cooling BTU and heating BTU. Both exist. Cooling BTU/hr is what an AC removes from the room; heating BTU/hr is what a heater adds. An appliance rated at 5,000 BTU cooling and 4,500 BTU heating is a heat pump with different efficiencies in each mode.
5. Ignoring altitude derating. At 5,000 feet elevation, air density drops about 17%, and both heating and cooling equipment output drops correspondingly. A 24,000 BTU/hr unit at sea level delivers about 19,900 BTU/hr in Denver.
Quick Reference: Common Wattages in BTU/hr
Memorize these for field estimation. 100 watts equals 341 BTU/hr. 500 watts equals 1,706 BTU/hr. 1,000 watts (1 kW) equals 3,412 BTU/hr. 1,500 watts (a typical space heater) equals 5,118 BTU/hr. 5,000 watts equals 17,060 BTU/hr. 10,000 watts equals 34,120 BTU/hr. 12,000 BTU/hr equals 3,517 watts equals 1 ton of cooling equivalent heat rejection.
For HVAC load estimation: a typical person at rest emits 400 BTU/hr. A laptop computer emits about 200 BTU/hr. A desktop computer under load emits 1,000 BTU/hr. A refrigerator averages 400 BTU/hr. Commercial kitchen equipment can emit 10,000+ BTU/hr per appliance. Sum all heat sources when sizing AC for habitable spaces.
Industry Standards and References
The exact conversion factor 1 watt = 3.41214 BTU/hr is defined by the International Table BTU (ISO 31-4 standard). The "thermochemical BTU" is slightly different (1 watt = 3.41443 BTU/hr) but the International Table value is universal in HVAC and electrical industries.
ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) publishes the definitive load calculation methods in the Handbook of Fundamentals. For commercial AC sizing, Manual N and Manual J (residential) from ACCA are the standard tools. AHRI (Air Conditioning, Heating, and Refrigeration Institute) certifies equipment ratings to ensure the BTU and SEER values on labels are verified and consistent across manufacturers.
Watts to BTU: HVAC and electrical units that talk past each other
HVAC manufacturers list cooling and heating capacity in BTU per hour, while electrical loads come in watts. The conversion is fixed: 1 watt = 3.412 BTU/hr, or 1 BTU/hr = 0.293 watt. This is a pure unit conversion, not affected by efficiency or system type.
Where confusion creeps in: the BTU/hr rating on an AC is cooling output. The watt rating on the same AC is electrical input. They are not the same; they are connected by the EER or SEER efficiency rating.
The formula and what it does
The conversion factor 3.412 comes from 1 watt-hour = 3.412 BTU, dividing both sides by an hour. EER (Energy Efficiency Ratio) of an AC = BTU/hr cooling output divided by watts electrical input.
Worked example
Scenario: 12,000 BTU/hr window AC, EER 11.2.
Cooling output: 12,000 BTU/hr = 12,000 x 0.293 = 3,516 W of heat removed per hour. Electrical input: 12,000 / 11.2 = 1,071 W. So the AC moves 3,516 W of heat out using 1,071 W of electricity. The COP (coefficient of performance) is 3,516 / 1,071 = 3.28, which matches the EER conversion exactly (EER / 3.412 = COP).
Common mistakes to avoid
undefinedFrequently asked questions
How big an AC do I need for a 400 sq-ft room?
Rule of thumb is 20 BTU/hr per sq-ft, so 8,000 BTU/hr. Adjust for sun exposure, ceiling height, and occupancy. ASHRAE Manual J does the rigorous version.
What is the difference between BTU and ton?
1 ton of cooling = 12,000 BTU/hr. A 3-ton AC moves 36,000 BTU/hr of heat. The unit comes from the heat needed to melt 1 ton of ice in 24 hours.
Why is electric heat 100 percent efficient?
All the electricity becomes heat (resistance heating). 1500 W in = 5,118 BTU/hr out. Heat pumps beat this by moving heat instead of making it, getting 250-400 percent equivalent efficiency.
What is the BTU rating of natural gas?
About 100,000 BTU per therm, or 1,030 BTU per cubic foot. Useful for comparing electric vs gas heating costs.
How does humidity affect BTU sizing?
Latent load (humidity removal) is roughly 25-30 percent of total cooling load in humid climates. Manual J accounts for it; rule-of-thumb 20 BTU/sq-ft assumes typical humidity.
Why do mini-splits list BTU but draw watts?
BTU is what they do (cooling output). Watts is what they consume (electrical input). Ratio = COP, multiplied by 3.412 = HSPF or SEER depending on test conditions.