What is the standard unit for measuring the quantity of heat in a substance?

The standard unit for measuring the quantity of heat in a substance is the British Thermal Unit (BTU). This unit specifically quantifies heat energy and is 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 heating and cooling systems, particularly in HVAC contexts, to describe the capacity of heating and cooling equipment.

While Watts refers to the rate of energy transfer (not a measure of heat itself), Joules are a more universal measurement of energy, which can include heat but is typically applied in broader energy contexts. Kilocalories, another measure of heat energy, are often used in dietary contexts to describe the energy content in food rather than in heating or cooling systems. Therefore, BTU is the most relevant and commonly utilized unit of measurement for heat in the context of air conditioning and thermal processes.