How to calculate electrical power in watts?

Teaching how to calculate electrical power in watts seems obvious, since electrical power is only given in watts, isn’t it? In fact it is not so, there are other types of electrical power and several ways to calculate. We will cover in this article what are the types of power and the differences between the other electrical powers.

It is common for people to think that there is only the electrical power  given in watts, called active power, because after all, what we usually learn in basic electrical courses is calculating power in resistors for example, or perhaps because we know that the power utilities charge us based on active power.

Power types

In alternating current circuits, voltage and electric current behave differently than in direct current circuits, in addition there is a lag between these two quantities of electricity , voltage and current. There are several differences between alternating current and direct current circuits, between the differences we have the electrical powers. In reality, in alternating systems there are three types of electrical power, which are:

Active power:

The active power is nothing more than the power that performs useful work on the load, and its unit of measurement is the watt (W). Active power is power that we actually pay for energy utilities.

Reactive power:

The reactive power represents part of the power that is used in the capacitive and inductive loads of the circuits without performing effective work, and its unit of measure is reactive volt amp (Var). This power is a small part of the total power consumed by the induction motors, which is necessary to generate an electromagnetic field, for example, if this power value is above the values ​​allowed by the concessionaire, a fine is charged to the consumer.

Apparent power:

We can define the apparent power as the total power that a given source is capable of delivering, that is, it is the “vectorial sum” of the active power and the reactive power, this being the total power that the energy utility can supply us . The unit of measurement of apparent power is the volt amp (VA).

How to calculate active power.

It is important to highlight that in addition to these three powers there is a factor that relates them, which is the power factor (φ), also known as angle fi. The power factor is a relationship between the apparent power and the active power, that is, it is the relationship between the amount of energy delivered by the source and the amount of energy that is effectively transformed into work, the power factor can be very high or very low, making it necessary to correct the power factor, that is, if the power factor of a machine is very low, a capacitor bank is installed, but if it is too high, a reactor bank must be installed.

Calculate power in watt:

Among the three types of powers we have just seen, we have heard a lot about active power and apparent power, especially for electricians who always work with electrical projects , because when developing an electrical project for a home, we use the apparent power and not the active power to perform the division and dimensioning of the circuits in an installation.

Considering a purely resistive load, in this case a resistor, we can find the value of its power using three very common formulas, which are represented in the image below:

Formulas for calculating active power.

It is very common to use the active power for dimensioning, so we will teach how to convert the power from VA to W. In order to do this conversion it is necessary to have the value of the power factor of each load and the value of the active power.

Apparent power for active power:

Finding the wattage value of a given load is very simple, we have to multiply the load value in VA by the power factor. We will calculate the apparent power of a load with a power factor of 0.94 and an apparent power of 800VA. See the resolution in the image below:

Converting apparent power to active power.

Active power for apparent power:

Finding the apparent power value is also very simple, as it is enough to have the value of the power factor and the apparent power of the load and perform the opposite calculation of the previous question, that is, terms that divide the value of the active power by the power factor . We will use as the basis for the calculations the same load as the previous example, which has an active power of 752W and a power factor of 0.94. See the resolution in the image below:

Converting active power to apparent power.