What is the primary way to measure heat transfer between systems?

The primary way to measure heat transfer between systems is in joules, as this is the standard unit of energy in the International System of Units (SI). Heat transfer, which is a form of energy transfer due to a temperature difference, is quantitatively expressed in joules.

While degrees Celsius are a unit of temperature and can indicate a temperature gradient that drives heat transfer, they do not measure the actual energy exchanged between systems. Watts represent power, which is the rate of energy transfer over time (one watt equals one joule per second), but they are not a direct measure of the total heat transferred.

Thus, while all three measurements—degrees Celsius, joules, and watts—are related to heat transfer in different contexts, joules specifically quantify the energy transferred. Hence, focusing on joules as the primary measurement leads to a clear understanding of heat transfer between systems.