kW or kWh/24

[mikec]

Hi,

I want to show the power consumption of a device over a 24hour period. So for example the device consumes .1kW

Should I write this as

a) 2.4kWh
b) 2.4kWh/24
c) 2.4kW

I want to convey to the reader that I am showing the power consumption for a 24period i.e.  I have worked it out and if they wish to see the power requirements of the device they can divide by 24.

Mike

[Simon]

why not just say that is uses 0.1KW ? most people can do 0.1 x 24 = 2.4

Otherwise you need to be a bit more verbal about it and say that it uses 2.4KW over a 24 hour period but unless your trying to clarify the average over 24 hours versus high instantaneous consumption that is less regular than every hour I can see the problem.

[Strada916]

1kWh is just that. if a device is rated at 1kW then it consumes 1kW per hour. so there is no need to say anything about 24hours. You can say the unit consumes 24kWhs  per day. or Daily consumption. as we all know a day is 24 hours. Does that make sense?   

[Rufus]

I want to show the power consumption of a device over a 24hour period.

Devices do not consume power in the same way that cars do not travel speed.

[IanB]

Hi,

I want to show the power consumption of a device over a 24hour period. So for example the device consumes .1kW

Should I write this as

a) 2.4kWh
b) 2.4kWh/24
c) 2.4kW

I want to convey to the reader that I am showing the power consumption for a 24period i.e.  I have worked it out and if they wish to see the power requirements of the device they can divide by 24.

Mike

You should write that the device consumes 100 W.

If the consumption varies, but averages out to 100 W over a 24 h period, you should write that the average power consumption is 100 W, but the peak power consumption may be as much as XXX W.

For example, my laser printer has a peak power consumption of 1000 W, but it doesn't use 1000 W continuously. The average is much lower and varies according to how much printing I do.

[IanB]

Devices do not consume power in the same way that cars do not travel speed.

You do not speak English in the way that I do not speak Russian.

In other words...huh? 

[c4757p]

In other words...huh? 

He's being pedantic and offering the correction that energy is consumed, not power.

[Rick Law]

In other words...huh? 

He's being pedantic and offering the correction that energy is consumed, not power.

Ahem, consumed implies destruction.  Energy cannot be destroyed, it can only be transformed.

[Psi]

If i had to write the number as a 24h total i'd probably write it like this..

Total power consumption is 2.4kW/day (100Wh)

The 100Wh in brackets is important because it calls attention to the deviation from normal notation.

[Kremmen]

Allow me to disagree. Power/time is certainly not the unit of energy. So instead of writing 2.4kW/day, you should really write 2.4kWh, because that is what you mean. Point being that the power and time are multiplied to get energy; not divided. So the right answer is emphatically not 2.4 kWdays because that would imply a continuous power of 2.4kW which is not at all the case.

The fundamental problem seems to be that (some) people have difficulty getting the difference between power and energy.
If it is too difficult to understand concepts like P_ave = 100W or E₂₄ = 2.4kWh then one just has to explain in more words.

[smashedProton]

I say to just leave it in watts if you are writing for engineers.  If you are doing narketing and spin doctor voodoo, that is a different question.  

Kilowatt hours are energy.  Do the dimensional analysis.

J/s=p. J/s/s = js/s=j

[Psi]

Allow me to disagree.

no

 

[Galaxyrise]

Allow me to disagree. Power/time is certainly not the unit of energy.

You are correct that Power/time is not energy.  (J/s)/s is J/(s^2), not J.  But when someone refers to using a kilowatt for a day, that's not P/t! That's P*t, which is energy.

The energy star labels we get here in the US tend to list things in kWh/year. For Example: "550 kWh Estimated Yearly Electricity Use" (source) The EnergyGuide spells it out to avoid scaring people with fractions, and it goes with "use", nicely avoiding Rick's concern over the verb "consume"

So I'd recommend "Uses 2.4kWh per day".

[mikec]

First off thanks to all for answers and apologies for not replying sooner..

So I should have been more more accurate, the reason for picking a timeframe is in fact I am creating a table which will show other variables (chemicals) that are consumed in the process cycle period (I said 24hours but it could be 10 to 15hours). So my table will be setup to show the total of everything consumed. I understand from the posts now that I shouldn't have said "power consumed" as correctly pointed out power cannot be consumed. Saying "energy consumed" is wrong also if your meaning of consumed is to be destroyed, but I never hear of people speaking about energy as been "converted" in normal conversation.

I agree that "Uses 2.4kWh per day" is the easiest and probably the best way to write this. I could just show this as "2.4kWh per cycle" where cycle is clearly indicated as been 15hours for example.

 

[IanB]

I understand from the posts now that I shouldn't have said "power consumed" as correctly pointed out power cannot be consumed. Saying "energy consumed" is wrong also if your meaning of consumed is to be destroyed, but I never hear of people speaking about energy as been "converted" in normal conversation.

I think here you were on the receiving end of an excess of pedantry. Power is useful when transmitted from one place to another and it is entirely reasonable to consider that power has generators at one end and consumers at the other. This in fact is completely standard engineering terminology. Don't let the nit-pickers confuse you 

[Rufus]

I think here you were on the receiving end of an excess of pedantry. Power is useful when transmitted from one place to another and it is entirely reasonable to consider that power has generators at one end and consumers at the other. This in fact is completely standard engineering terminology. Don't let the nit-pickers confuse you 

The sloppy and incorrect use of the word power to describe energy is why there is widespread confusion and incorrect usage of the units of both.

Power is a rate. It is energy per unit time. You can not transmit a rate you can not measure the quantity of anything in units of rate.

If you go to a party and someone asks how far did you drive to get here and you answer 43 miles per hour you would be thought a loon yet when it comes to power and energy I hear and see the equivalent all the time.

[IanB]

Power is a rate. It is energy per unit time. You can not transmit a rate you can not measure the quantity of anything in units of rate.

Yes indeed. Some very strange things must be happening when my car transmits 240 bhp from the engine to the wheels through the transmission. Some very fishy things must be going on there, I'm sure of that...

Perhaps my car doesn't move at all and it's all an illusion?

[Rufus]

Power is a rate. It is energy per unit time. You can not transmit a rate you can not measure the quantity of anything in units of rate.

Yes indeed. Some very strange things must be happening when my car transmits 240 bhp from the engine to the wheels through the transmission. Some very fishy things must be going on there, I'm sure of that...

It doesn't transmit power it transmits torque which results in the transmission of energy when the shaft or wheels turn. Power is the rate at which energy is transmitted it is not what is transmitted.

If you think it transmits power then tell me how many bhps it has transmitted after driving for 15 minutes.

[IanB]

It doesn't transmit power it transmits torque which results in the transmission of energy when the shaft or wheels turn. Power is the rate at which energy is transmitted it is not what is transmitted.

If you think it transmits power then tell me how many bhps it has transmitted after driving for 15 minutes.

Well if it transmits torque, can we measure how much torque it has transmitted after driving for 15 minutes?

[Mar]

WH  its energy capacity or in vice verse its the consuming from energy per hour this mean that 1KWh give 1 KW through one hour
  .. when you are talking about POWER  .. its just KW ... or W ...
 
so when u say the Device consume 0.1 KW ...  generally speaking it will be in hour ...
suppose that you measure it and yo find the device take 0.1KW..   . this mean it will take 0.1KWh .. in 1 hour 

so i think the answer is
0.1  KWh  ... in one day will be 4.2 KWh / day

[digsys]

so i think the answer is
0.1  KWh  ... in one day will be 4.2 KWh / day 

OOHH NOOO !! Please correct you're multiplication result or there'll be another 20 nit-pick posts !! :-) OH NOOO I've used the wrong your ...
<runs for the door> hehehehe

[Galaxyrise]

I do say my car "travels speed".  I say it "travels 60mph".  Perhaps "travels at 60mph" would be more correct, but I expect the units to disambiguate the sentence.  Just like if I say a cable transmits 100watts, I expect the listener to understand that it is transmitting energy at the rate of 100J/s. 

You can not transmit a rate you can not measure the quantity of anything in units of rate.

I don't know what you're saying here, as rates are measured all the time.  Do you measure constant current power supplies in amps?  That seems like the only way to measure one.  But an amp is a rate. Consider the term transmission line.  Whether you're talking about transmitting frequency, current, or power over one, they're all rates.

It is good to know the distinction between energy and power. But thinking in terms of, and measuring in rates is often useful and natural.

And I don't object to the term "consume", either.  To consume is to "eat, ingest, buy, spend".  Not "destroy".  And the verb has context.  Just like when I talk about consuming water, I don't mean that my body is destroying water.  I mean I'm taking it from the clean water reservoir and it's not available for someone else to use directly.  In the case of consuming electrical energy, it's being used by a device to do some work, and now nobody else can use that energy for their own devices.

[mikes]

a) 2.4kWh
b) 2.4kWh/24
c) 2.4kW

8.64 MJ, which avoids all the power/energy confusion which comes from using a unit of power to specify energy. It's the right way, being the SI unit for energy. kWh is like specifying Amp-Ohms instead of Volts.

[Kremmen]

a) 2.4kWh
b) 2.4kWh/24
c) 2.4kW

8.64 MJ, which avoids all the power/energy confusion which comes from using a unit of power to specify energy. It's the right way, being the SI unit for energy. kWh is like specifying Amp-Ohms instead of Volts.

Sure but try to get Joe Average's head around that and see how far you get. Cars should move in m/s and the engines should rev in radians/s and engine power should be in kW but no. Tradition rules. And kWh does have the advantage of being an "understandable" unit in the sense that one can roughly visualize the kind of energy level 1 kWh represents. I am a fully paid member of the SI fan club and maintain that all calculations are to be done in pure SI units only. It is both easier and less error prone and not so many probes crash on Mars as a result. But when "laymen" are involved one should talk the same language, i.e. "theirs". The energy meter on the wall is calibrated in kWh so pulling the rug and suddenly introducing MJ is - while corret - also needlessly confusing.
I think the OP already arrived at the best compromise, i.e. x kWh / cycle. That is actually quite understandable IMO.