Electric power and energy measurements are probably the most important and critical measurements in power systems, since they are used to quantify every energy transactions and assign an economical value to energy flowing through a given section of the grid. Things became even more complex when the first AC distribution systems began to be used as an alternative and more efficient way to distribute electricity and, in the end, replaced the DC systems almost completely. It is generally thought that power and energy definitions are the same for DC and AC circuits. Under time-varying conditions, electric and magnetic fields exist in the same point of space and are related by Maxwell equations. Wattmeters and energy meters are probably among the oldest instruments built since the industrial exploitation of electric energy. Power and energy measurements are performed in power systems at all voltage levels, from low-voltage to ultrahigh-voltage systems.