A British Thermal Unit (BTU) is a traditional unit of heat energy, defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. BTUs are commonly used in North America to rate heating and cooling equipment capacity—a furnace might be rated at 80,000 BTUs, indicating how much heat it can produce per hour. Natural gas is often sold in units related to BTUs. While the metric system uses joules and calories for energy, BTUs remain prevalent in Canadian heating, ventilation, and energy discussions due to the shared market with the United States.