Power Rating

Do you know why your mum or dad is constantly reminding you to turn the lights off when you leave your room? It might be to reduce your electricity bill, but have you wondered how the electricity department decides the amount you pay every month? The electrical appliances that we use in our everyday lives draw a current from the national grid when they are operated that depends on their power rating. This is an energy transfer taking place from the grid to power the devices at home. For example, an electric kettle converts electrical energy into heat energy that heats up the water for our morning coffee or tea.

In this article, we will discuss how electrical energy is transferred between appliances and the factors it depends on. We will also look into how the power consumed by an electrical appliance is calculated and work on a few examples. You will also be able to make informed decisions when you're out shopping for any new electrical device. So keep reading till the end to find out - oh, and don't forget to turn your lights off!

Power ratings definition

Power ratings are what we see on our home appliances, defining how much energy is being transferred from the mains to power the device. The power rating helps consumers choose between different appliances based on their power consumption. It also highlights the maximum power at which the appliance can safely operate, which the cable and plug also need to be able to handle.

Not all the electrical energy transferred into an appliance is converted into useful work. A part of the energy is almost always converted into heat energy or some other form of waste in electrical devices. The efficiency of the appliance tells us how much of the input energy is converted into useful work. We'll speak more about the term efficiency and what it means in a later section.

Power rating formula

The Electric power rating formula in a circuit can be calculated using the formula:

$P=VI$

or in words:

$Power=Electricpotential\times Current$

There is also another method to calculate the power rating of an appliance. It can also be calculated using the work done or energy transferred by an appliance in a given amount of time.

$P=\frac{W(\mathrm{or}E)}{t}$

or in words,

$Power=\frac{workdone}{Timetaken}$

Here$W$is the work done or$E$the energy transferred in joules, and$t$is the time in seconds. The power rating unit is watts. For appliances that consume higher values of power, we use kilowatts$({10}^3\mathrm{watts})$or megawatts$({10}^6\mathrm{watts})$.

From the above equations, we can see that the power consumed by an appliance depends on the total energy transferred and the time the appliance is on for. The above equation can be re-arranged to obtain the energy consumed by an appliance. Another way to calculate the rate of energy transfer to an appliance is by measuring how many coulombs of charge flow through a given potential difference. This is given by:

$E=QV$

or in words,

$\mathrm{energy}\mathrm{transferred}=\mathrm{charge}\mathrm{flow}\times \mathrm{potential}\mathrm{difference}$

where$Q$is the charge in coulombs$\left(\mathrm{C}\right)$and$V$is the potential difference in volts$\left(\mathrm{V}\right)$. Now lets look at the efficiency; this will not only help you in your GCSEs but also when you're out shopping for any new electrical appliance.

Power Rating of appliances

When you turn on an electric device it is designed to convert the electricity into whatever useful work it was intended to perform. During this conversion, some energy is always lost, typically as heat or noise. An efficient device is one that minimizes this energy loss. So if we have two devices with the same power consumption rating, examining their efficiencies tells you how much of the power consumed is being converted into useful work. Efficiency can be calculated as follows.

$\mathrm{efficiency}=\frac{\mathrm{useful}\mathrm{output}\mathrm{work}}{\mathrm{total}\mathrm{input}\mathrm{energy}\mathrm{transfer}}$

It can also be calculated as

$\mathrm{efficiency}=\frac{\mathrm{useful}\mathrm{power}\mathrm{output}}{\mathrm{total}\mathrm{power}\mathrm{input}}$

Calculating efficiency will give you a decimal value less than or equal to one - a useful way to represent this is by using a percentage. When you multiply the efficiency with$100\%$, we get what is called a percentage efficiency. A theoretical device with$100\%$efficiency converts all the supplied power into useful power. A percentage efficiency of$20\%$means that the device is only converting$20\%$of the supplied power into useful power or work.

Now let's work on a few examples to practice what we just learned.

Power rating examples

Power rating of the resistor

Different resistors have different power ratings, which define the maximum power that can pass through the resistor without damaging it.

Every resistor comes with a specific power rating. A resistor gets heated up as it obstructs the flow of current through it. Thus if a resistor has a maximum power rating, it is to prevent it from heating beyond the limit of how much heat it can dissipate. The power rating of the resistor is usually measured in watts.