SI Derived Units: Power, Radiant Flux, and the Watt

Power and radiant flux use the named SI derived unit of the watt, named for James Watt, first established in 1908 (though it has since been redefined). Power can be defined as the rate at which work is performed, or the amount of energy transferred per unit of time. The watt is equivalent to one joule per second, or in base units kg m^2 s^-3.

Mathematically the watt is represented by the capital letter W; power is often represented by a capital P and radiant flux by the capital Greek letter phi with the subscript of a lowercase e. One of the only other named units of power is the horsepower. Many other unit systems simply use their values for energy over time (ergs per second, BTUs per hour, etc.).

Sources/Further Reading: (Image source - Energy Purse) (Wikipedia: Watt, Power, Radiant flux) (Metric System) (Khan Academy)

Enthalpy

Mathematically, enthalpy is represented by the capital letter H and is equal to the internal energy plus the product of the pressure times the volume with units of energy (in SI units it is the joule [J]). It is often a quantity referenced in the amount changed, i.e. what is the change in enthalpy, as this change represents the heat that is either absorbed or released during a process. Enthalpy therefore depends on the temperature, pressure, and volume of a substance and can be used to determine if a process is exothermic (releases heat) or endothermic (absorbs heat). As a state function, the change in enthalpy is independent of path: the enthalpy required to melt a solid at its melting temperature (a specific term known as the enthalpy of fusion), for example, is a constant value dependent only on the material.

Sources/Further Reading: (Image source - NASA) (University of Bath) (University of Central Arkansas) (LibreTexts) (Wikipedia)