Electrical power: what it is and how it is calculated
Do you want to know how electrical power is calculated? In our guide, you will learn the power formula, practical examples and much more.
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Understanding electrical power and how it is calculated is essential to promote efficient energy use and minimize environmental impact. The fact is that, with the immense dependence on technology and electricity in our lives, knowledge of these concepts becomes a valuable tool for consumers and professionals in the electrical sector. Proper management of electrical power contributes not only to the optimization of resources, but also to long-term sustainability!
1 Meaning of electrical power
We have all heard about “electrical power” at some point, but do we really know what it is? As a fundamental concept in the field of electricity, electrical power plays a crucial role in our daily lives and in the operation of numerous devices and systems.
Electrical power is defined as the rate at which work is done or energy is transferred in an electrical circuit. In other words, it represents the amount of energy consumed or supplied by an electrical device in a given period of time.
2 Power Types
Going deeper into this guide, we must keep in mind that electrical power is classified into two types: active (or real) power and reactive power. Active power is that which actually performs useful work, while reactive power is related to the transfer of energy between inductive and capacitive components of the circuit, without performing direct work.
What better than to go deeper with examples? Active power is the spark that powers direct, useful actions, such as turning on a light or moving a vehicle, while reactive power is more related to the temporary storage of energy, ready to be used in devices that need a momentary boost without doing direct work at the time.
3 Electric power formulas
In this section we are forced to get mathematical, but don't be scared, later we will put it into practice with more practical examples. The first piece of information you need to know is that the unit of measurement for power in the International System is the watt (W).
The calculation of electrical power is based on Ohm's law and Joule's law. The fundamental formula to calculate power (P) is:
where:
- P is the power in watts (W).
- V is the potential difference or voltage in volts (V).
- I is the current in amperes (A).
- cos(θ) is the power factor, which indicates the efficiency in energy conversion.
In direct current (DC) circuits, the calculation is simpler, since the power factor is not considered.
The formula simplifies to:
P=V⋅I
In the case of alternating current (AC) circuits, both active and reactive power must be taken into account.
The total power is expressed as:
S=P+jQ
where:
- S is the apparent power in volt-ampere (VA).
- P is the active power in watts (W).
- Q is the reactive power in volt-ampere reactive (VAR).
- j is the imaginary unit.
Remember that optimizing power factor is essential to improve energy efficiency and reduce losses in the electrical system!
4 Practical examples
Electrical power on the electricity bill
To calculate the electrical power in the electricity bill, we would need to know both the electrical energy consumption (expressed in kilowatt-hours, kWh) and the contracted power (expressed in kilowatts, kW). The contracted power is the maximum amount of energy that can be consumed simultaneously in a home or business and is established when contracting the electrical service with the supply company. Let's assume that on your electricity bill, you have consumed a total of 300 kWh during the month. In addition, the contracted power for your home is 5 kW. To calculate the average electrical power (or active power), we can use the formula:
| Electrical Power (kW) = | Energy Consumption (kWh) |
| Hours in the Month |
It is important to keep in mind that the result will be in kilowatts (kW). If we assume that there are 30 days in the month and that each day has approximately 24 hours, we can calculate the electrical power as follows:
| Electrical Power (kW) = | 300 kWh |
| 30 days x 24 hours/day |
| Electrical Power (kW) = | 300 kWh |
| 720 |
| Electrical Power (kW) = | 0.4167 kW |
So, the average electrical power in your home is approximately 0.4167 kW.
Now, to verify if the contracted power is sufficient, we can compare it with the contracted power of 5 kW. In this case, the contracted power is more than enough to cover your average demand, since 0.4167 kW is significantly less than the contracted power of 5 kW.
Electrical power in everyday devices
Let's imagine a simple scenario in which we want to calculate the electrical power used by a lamp in a home. For this, we need to know two key factors: the voltage (V) at which the lamp operates and the current (I) that flows through it.
Suppose the lamp operates at 120 volts and the current passing through it is 1.5 amperes. As we already saw in the previous section, to calculate the active (real) power, we use the formula:
P=V⋅I
P=120V⋅1.5A
P=180W
The active power of the lamp is 180 watts. This means that the lamp consumes 180 watts of electrical energy while on.
Now, suppose we want to take into account the apparent power, which includes both active and reactive power. Suppose that, in this case, the power factor (cos(θ)) is 0.9. The formula for apparent power (S) is:
S=P/cos(θ)
We substitute the known values:
S=180W/0.9
S=200VA
So the apparent power of the lamp is 200 volt-ampere. This figure includes both active and reactive energy and gives us a more complete picture of the electrical performance of the lamp in the circuit.
Note that this practical example illustrates how to calculate electrical power in a simple device, taking into account both active and apparent power. You can continue inventing situations and calculating the power.
