For power measurement, it is important to look at energy first. In a simple system, you have a signal source and a load. Now you can transfer electric energy from the signal source to the load. This energy can be absorbed or converted into another energy, such as light, heat, rotation, etc. This is called active energy. In addition, it can be stored and returned to the signal source. This is called reactive power.

Each energy transmission determines the success rate. Therefore, you can transmit power to the load within a specified time. Some of this power may return to the signal source (reactive power), while the other part will not return (active power).

Electricity can be divided into four groups:

Instantaneous power: This is the power at a certain moment. This is calculated by multiplying the instantaneous voltage and current at that time point. A series of instantaneous power values produce power oscillation fluctuations. The character representation of this ability is "p(t)". So the formula is:

Apparent power: This is the product of the true effective value of voltage and current. Signals with phase shift or distortion are not considered. Its formula is:

Active power: this refers to the power transmitted to the load and not returned within the specified time. So the active power is the average value of power oscillation. Its formula is:

The sign of p is "+"or "-",depending on the direction of power flow. Active power can only be generated by voltage and current with the same frequency.

Reactive power: This refers to the power delivered to the load and returned within the specified time. The calculation formula of reactive power is: