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## Class 9 Physics (India)

### Course: Class 9 Physics (India) > Unit 4

Lesson 6: Commercial unit of energy# Commercial unit of electrical energy

Let's explore the unit of electrical energy (kiloWatthour) that we use for commercial purpose. Created by Mahesh Shenoy.

## Want to join the conversation?

- when power is 500w then why is it 1/2(1 vote)
- 1/2*500 is250 then why is it 0.5kwhr(2 votes)

## Video transcript

we pay money to electrical company for providing us the electrical energy and if you ever looked at the receipt or back of the receipt then you can actually see how much money you pay for every unit of energy that you consume right let's mention all over here but the question over here is what is the unit of energy that they are referring to is it joules or is it something else so that's what we want to explore in this video the unit of energy we use commercially like the electric companies do is called the kilo watt R so in this video we'll explore what this kilowatt R is and we also talk about why we don't use the standard unit of energy the jewels why we don't use that why do we talk about this so what's a kilo watt R to begin with we can see there is energy over here there is kilo watt over here what is the unit of power and R is the unit of time so let's quickly recall the connection between power energy and time we have seen in previous videos that power consumed by any device electric power consumed by any device is usually written as the amount of energy that it consumes per time right and so from this since we want to know what energy is because you are looking at the unit of energy from this we can now write energy is power multiplied by time now if we substitute for power in kilowatt so let's say we have a device which has a power of 1 kilowatt and if we switch it on for 1 R see you put time as 1 R then the energy that that device would have consumed by definition as you can see is 1 kilowatt R that make sense so if let's take an example what I mean to say is imagine we have an air-conditioner let's say we have an air-conditioner as an example and if the power rating of this air-conditioner is one kilowatt thousand watt and if this air conditioner is switched on for one hour so if we switch this on for one hour then the energy consumed by this air conditioner is one kilowatt hour and so our electric companies just call it as one unit so whenever they say unit they mean kilowatt our let's take another example suppose we have let's say a refrigerator a refrigerator which has a power rating of this door handle okay which has a power rating of let's say 500 watts I think refrigerators usually have a lower power I think what anyways it's an example and let's say we use it for 10 hours so how what is the energy consumed in commercial units well we do the same thing we see we say energy is power into time we substitute for power but not in watts we have to substitute in kilowatts now 500 watt is half a kilowatt right kilowatt is thousand so this is half of that so we'll substitute half a kilowatt or 0.5 kilowatt it's rather than 0.5 so here's 0.5 kilo watt and time is 10 hours so if you put 10 hours what we end up with now is that 0.5 times 10 is 5 kilowatt are 5 kilowatt are so we'll just call it as 5 units 5 units so commercially speaking people would say in this scenario your refrigerator has consumed five units of energy over here we would say our air conditioner has consumed one unit of energy all right so why do we introduce this new unit of energy why can't we just work with joules well simply because it turns out that a kilowatt R is a very kind vini n't unit of energy for electric companies to put a price on turns out that Joule is a very very tiny unit of energy it's extremely inconvenient to put a price on every Jew that they're going to supply or they're going to provide for example I'm pretty sure you're familiar with this concept if you take the example of of salary that people get if you're working for a company then the company pays you for the amount of time you put in now the standard unit of time is seconds but you don't put a price on every second you work right you don't go and say that you know you pay me one rupee for every second I work that is so weird nobody says that why because second is just a very tiny unit of time for us in in this context and so forth for that we we look for a larger unit of time and and that's ours usually we choose ours so we can say like you know for every hour I put in you pay me 500 rupees or thousand rupees that's usually what people do so the same idea over here for convenience so now we might be wondering well how big is a kilowatt are compared to joules well for that all we need to do is convert this into joules and to do that we have to just put the power and the time in standard units the standard unit of power is what the standard unit of time is seconds so let's put one kilowatt are in standard units and see what we get all right so let's do that somewhere down over here so if we take one kilowatt R that's going to be one kilowatt is a thousand watts right that's a thousand watts times time is one R let's put one R in terms of seconds because second is a standard unit that's going to be 3600 seconds so that's going to be let's see one two three four five so that's going to be thirty six and one two three four five joules okay so that's roughly three point six then there are five zeros here this one more so we'll take three point six times ten to the power six joules oh that's three point six million joules and that's an extraordinary big number right compared to joules so you can see one kilowatt R or one unit commercial unit is a significantly larger chunk of energy compared to a jewel and roughly for every kilowatt R that we consume every unit of energy we use we consume our electric company charges somewhere around three to six rupees of course this this varies with from place to place and it also varies based on how much how much electricity you're consuming but roughly it's around this number I mean we can go back and look at that receipt if you look at this receipt we can actually see the price listed over here so the price is the amount of money that we're paying per unit and that unit is none another none other than the kilowatt R and you can see it's not a it's not a constant keeps varying you can see it's also becoming it can also be more it depends upon what you're using it for so whether it's household or we're using it for industry or proper purpose there you are ruler urban areas so it depends upon also how much energy you consume in anything and so we are paying less than 10 rupees for three point six million joules so can you imagine how much we are paying for one Joule that's gonna be such a small number it's going to be some point zero zero one pay say or something like that and that's what you'll see is so inconvenient and that's why we prefer kilowatt R so to summarize a kilowatt R is the commercial unit of energy usually we just call it as units and to calculate energy in kilowatt R or in units we need to make sure that the power is in kilowatts and the time is always in ours and one kilowatt R is three point six million joules and one small note is whenever I would hear this word kilowatt R I would immediately think it's the unit of power but it's not the rid of power remember kilowatt is the unit of power but over here we are multiplying the kilowatt with time and when you multiply with time it becomes unit of energy so kilowatt R is not the unit of power all right so please don't make the same classic mistake that I would do