BTU
Definition
British Thermal Unit (BTU) is a traditional unit of heat energy still widely used in the US for heating, cooling, and energy measurement. One BTU is the energy required to raise the temperature of one pound of water by one degree Fahrenheit. Air conditioners are rated in BTU/hour (a 12,000 BTU/hr unit = 1 ton of cooling). Furnaces are rated by BTU input (80,000–120,000 BTU for residential). Water heaters are rated by first-hour BTU delivery. Conversions: 1 BTU = 1,055 joules; 1 kWh = 3,412 BTU; 1 therm (natural gas) = 100,000 BTU. Rule of thumb for AC sizing: 20 BTU per square foot of living space.
Why is BTU Important?
In electrical engineering and everyday applications, BTU is a fundamental concept for understanding how electrical systems work. Whether you are an engineer designing circuits, an electrician sizing wires, or a homeowner estimating energy costs, this metric is essential for safety, efficiency, and accurate calculations.
Our electrical conversion calculators help you quickly convert between units and verify calculations, reducing errors and saving time in both professional and DIY electrical work.
What is a BTU?
A BTU (British Thermal Unit) is a traditional unit of heat energy defined as the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. BTUs are the standard unit for rating HVAC equipment, water heaters, furnaces, and fuel energy content in the United States.
Common BTU Ratings
| Equipment | BTU Range | Use |
|---|---|---|
| Window AC unit (small room) | 5,000–8,000 BTU | 150–350 sq ft |
| Window AC unit (large room) | 10,000–15,000 BTU | 400–700 sq ft |
| Portable space heater | 5,000–10,000 BTU | 200–400 sq ft |
| Tankless water heater | 120,000–199,000 BTU | Whole house |
| Gas furnace (residential) | 40,000–120,000 BTU | Whole house heating |
| Central AC (1 ton) | 12,000 BTU | ~500–600 sq ft |
| Gas stove burner | 5,000–18,000 BTU | Cooking |
| Gas grill | 25,000–60,000 BTU | Outdoor cooking |
| Gas fireplace | 20,000–40,000 BTU | Supplemental heating |
BTU Energy Content of Fuels
| Fuel | BTU per Unit |
|---|---|
| Natural gas | ~1,030 BTU per cubic foot |
| Propane (LPG) | ~91,500 BTU per gallon |
| Heating oil (#2) | ~138,500 BTU per gallon |
| Gasoline | ~124,000 BTU per gallon |
| Electricity | 3,412 BTU per kWh |
| Cord of wood (hardwood) | ~20–24 million BTU |
AC Sizing by Room Area
General rule of thumb: 20 BTU per square foot for average room conditions, then adjust:
| Room Size | Recommended BTU |
|---|---|
| 100–200 sq ft | 5,000–6,000 |
| 200–300 sq ft | 7,000–8,000 |
| 300–450 sq ft | 9,000–12,000 |
| 450–700 sq ft | 12,000–14,000 |
| 700–1,000 sq ft | 18,000–24,000 |