# Kilowatt Hour >>> (Watt-the-f!% ) - Can someone please explain this doub to me ? :)

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#1
If a device transfers has a power transfer rating of 15kW in 20 minutes, does it have a kilowatt hour of 5 kWh, or 45 kWh ?
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3 years ago
#2
The watt is the unit of power, which is the rate at which energy is transferred or transformed. One watt is one joule per second; the joule is the standard unit of energy but is awkwardly small for most real-world measurements. A kilowatt-hour is the amount of energy transferred, when energy is transferred at a rate of one thousand watts over an hour - a much larger and more useful unit of energy. So the device transfers energy at a rate of 15 kilowatts, but only does so for 1/3 of an hour, so it has transferred 5 kWh.

I hope this helps
Last edited by anosmianAcrimony; 3 years ago
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#3
(Original post by anosmianAcrimony)
The watt is the unit of power, which is the rate at which energy is transferred or transformed. One watt is one joule per second; the joule is the standard unit of energy but is awkwardly small for most real-world measurements. A kilowatt-hour is the amount of energy transferred, when energy is transferred at a rate of one thousand watts over an hour - a much larger and more useful unit of energy. So the device transfers energy at a rate of 15 kilowatts, but only does so for 1/3 of an hour, so it has transferred 5 kWh.

I hope this helps
Hi there,

If the device has a power rating of 15 kW, doesn’t that’s mean it transfers 15 kW per second, as a Watt is a joule / second ? What about the value “15 kW” indicates it is per hour ?

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3 years ago
#4
(Original post by lewis.h)
Hi there,

If the device has a power rating of 15 kW, doesn’t that’s mean it transfers 15 kW per second, as a Watt is a joule / second ? What about the value “15 kW” indicates it is per hour ?

The device has a power rating of 15 kW, which means that it transfers 15 kilojoules per second. As you say, a watt is a joule/second. We never talk about power, or watts, being transferred - power is the rate at which energy is transferred.

So one watt is one joule/second. Multiplying by the number of seconds in an hour, we can also call one watt 3600 joules/hour. This is very similar to saying, okay, this object is moving at X miles per second, how fast is it moving in mph?

In any case, because 1 watt is 3600 joules/hour, 1 watt-hour is equal to 3600 joules and we can use watt-hours as a unit of energy. Again, when we say ''watt-hour'', we mean ''the amount of energy transferred in an hour when energy is transferred at a rate of 1 watt''.
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3 years ago
#5
(Original post by lewis.h)
If a device transfers has a power transfer rating of 15kW in 20 minutes, does it have a kilowatt hour of 5 kWh, or 45 kWh ?
A device has a power rating full-stop. It doesn't have one 'in' a given time - that would be an energy transfer.

Power is the rate of energy transfer, so energy = power * time. In this case, you want time in hours, so E=Pt=15kW * 1/3h = 5kWh.

You can think of it as speed (power) and distance travelled (energy transfered). If a car does 15mph, how far does it travel in 20 minutes?
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#6
(Original post by RogerOxon)
A device has a power rating full-stop. It doesn't have one 'in' a given time - that would be an energy transfer.

Power is the rate of energy transfer, so energy = power * time. In this case, you want time in hours, so E=Pt=15kW * 1/3h = 5kWh.

You can think of it as speed (power) and distance travelled (energy transfered). If a car does 15mph, how far does it travel in 20 minutes?
Hi there,

What I am strugglingto understand is what if it has a power rating of 15kW, this means it transfers 15000 J / second. So why are you treating the value like it is "/ hour" ? Surely, you should only do that if it was in "kWh", as opposed to just "kW" ?

Thanks.
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3 years ago
#7
(Original post by lewis.h)
What I am strugglingto understand is what if it has a power rating of 15kW, this means it transfers 15000 J / second. So why are you treating the value like it is "/ hour" ? Surely, you should only do that if it was in "kWh", as opposed to just "kW" ?
The KWh isn't a nice unit IMO - it's 3.6MJ. It is a convenient unit for consumers to calculate the cost of electrical energy. You could use MJ, but consumers prefer simpler maths, e.g. a 3kW electric fire uses 3kWh in an hour, and 6kWh in two. Multiply by the cost of a KWh of electricity, and you know what it costs to run.

I took the power (15kW) and multiplied by the number of hours (1/3 h - perhaps the lack of a space was an issue - the h is not on the denominator) to get to KWh.I'm not treating it as a per hour number, but the energy transferred is for a given period of time. You can think of power (the rate of energy transfer) as speed, and energy transferred as distance travelled. The same power for longer transfers more energy.
1
3 years ago
#8
(Original post by lewis.h)
If a device transfers has a power transfer rating of 15kW in 20 minutes, does it have a kilowatt hour of 5 kWh, or 45 kWh ?
Let's phrase this better: "If a device has a power rating of 15kW, how much energy does it use in 20 minutes?"

We could use Joules: 15KW * 1200s = 15KJ/s * 1200s = 18000KJ = 18MJ

Or we could use KWh: 15KW * 1/3 h = 5KWh

1 KWh = 1KJ/s * 3600s = 3.6MJ, so you could do the calculation in Joules and convert, if that's clearer for you.
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#9
(Original post by RogerOxon)
The KWh isn't a nice unit IMO - it's 3.6MJ. It is a convenient unit for consumers to calculate the cost of electrical energy. You could use MJ, but consumers prefer simpler maths, e.g. a 3kW electric fire uses 3kWh in an hour, and 6kWh in two. Multiply by the cost of a KWh of electricity, and you know what it costs to run.

I took the power (15kW) and multiplied by the number of hours (1/3 h - perhaps the lack of a space was an issue - the h is not on the denominator) to get to KWh.I'm not treating it as a per hour number, but the energy transferred is for a given period of time. You can think of power (the rate of energy transfer) as speed, and energy transferred as distance travelled. The same power for longer transfers more energy.
I see !

Thank you.
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