Any electrical appliance is characterized by several basic parameters, including rated voltage, current and power. Sometimes only the power and voltage are indicated in the technical data sheet , the current in this case is easy to find using Ohm's famous formulas (of course, with a number of reservations - for example, cos should be known). The converse is also true: knowing the current and voltage, you can perform a power calculation. The global web has a lot of materials on this topic, but most of them are designed for specialists.
Let's look at what is meant by the term "electric power", what its varieties exist and how the power calculation can be done. The physical meaning of power indicates how quickly the installation (device) converts electricity into one form or another of useful work. That's so simple! For non-electric devices, it is quite acceptable to use the term “performance”.
In electrical engineering, a division is accepted according to which there is active and reactive power. The first is directly converted into useful work, so it is considered the main one. The unit of measure is Watt and derivatives - Kilowatts, Megawatts, etc. It is indicated on household electrical appliances. Although this does not mean that there is no reactive component. In turn, the second is undesirable, since it does not participate in the work, but is wasted on various types of losses. Measured in "var" (volt-ampere reactive) and derivatives - kilovolt-ampere reactive, etc. The sum of the active and reactive components form the total power (volt-ampere, VA).
A striking example of a consumer with a net active load is an electric heater. When an electric current passes through it, heat is generated, and in direct proportion. The reactive energy consumer, the classic transformer, acts in exactly the same way. When it works, a magnetic field is created in the windings of the winding, which itself is not necessary (the property of electromagnetic induction is used). The magnetic core is magnetized, losses occur. In other words:
Q = U * I * sin Fi,
where sin Fi is the sine of the angle between the current and voltage vectors. Its sign depends on the nature of the load (capacitive or inductive).
The power calculation begins with determining the type of current: constant or variable, since the formulas are not universal.
In the first case, the corollary of Ohm's classical law is used. Power P is the product of current I by voltage U:
P = I * U (W = A * B).
When a circuit with a power source, the direction of the EMF is taken into account: this is necessary to calculate the resistance of the source itself. So, a generator or battery in which current flows from “-” to “+”, giving energy to the load of the circuit, gives off power. If the current flow is opposite to the applied potential (charging the battery), then the power is absorbed by the emf source.
The formula for calculating power for alternating current (single-phase circuit) takes into account the coefficient - "cosine phi". It represents the ratio of the active component of power to full. Obviously, in the case of a heating element, the cosine will be 1 (ideal), since there is no reactive component. Otherwise, various compensators or other technical solutions are used to reduce losses on the generator side.
Thus:
P = U * I * cos Fi.
Calculation of power in three-phase circuits is performed for each phase, and the obtained values are then summed. For alternating current, the total power is calculated as the square root of the sum of the squares of the active and reactive components. For generating devices (substations), it is more important to know exactly the total power, since on the basis of this, all other elements of subsequent circuits are selected. It is obvious that in most cases it is impossible to know in advance the nature of the load.