What unit measures internal energy and heat?

The best unit for measuring internal energy and heat in this context is the British thermal unit (Btu). The Btu is specifically defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is commonly used in the United States to quantify energy in systems involving heating and cooling processes.

While other options like the calorie and joule also measure heat and energy, the context of the question suggests a focus on units familiar in certain industries and practices, particularly in the United States, where the Btu is more prevalent. The joule is part of the International System of Units (SI) and is used globally, making it a valid measure for energy, but the Btu is typically preferred in specific applications related to thermodynamics and heat transfer.

The watt, on the other hand, measures power, which is the rate of energy transfer and not directly related to internal energy itself. Thus, the selection of the Btu aligns with common practices in measuring heat energy in specific contexts.