JayCan
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Re: Why do we still confuse KW and KWH?

Thu Aug 29, 2019 6:49 am

I'm a newbie to Leaf ownership and this forum so I have not read all the connected streams. However, I really like this thread and will chime in my 2 cents.
The discussion of Kw and Kw/h sometimes is 'the cart before the horse' and I find it easier to understand when it's approached as an energy conversion (energy cannot be destroyed, just changes state) and thinking of kilowatts as an end product, not a stand alone unit.

From a mechanical point, to rotate something takes 2 things; torque (in ft/lbs or N/m) and rotational speed (RPM). This can be converted into a horsepower (HP) rating (thank you James Watt). HP can then be converted into watts (1 HP = 746w) which can then be converted into electrical energy terms known as 'true power' (I am not going to bring in apparent and reactive power as that is a whole other component that can be discussed later).
For heat energy, one Kilowatt of heat per hour = 3415 BTU/hr
From a chemical point, batteries use a chemical reaction to create an electrical charge at a certain rate (Amp/hour) and to charge them takes a certain rate of charge.
In electrical terms watts are a product of Voltage (E) and Current (I) - W = E x I (coulombs got mentioned earlier :) )
Let's say a charger uses 10 amps at 120V - 120V x 10A = 1200w or 1.2Kw
If it uses the 1.2Kw constantly for 1 hour then the rate of usage is 1.2Kw/hr. If your car takes 10 hours to charge - 1.2Kw/hr x 10 = 12Kw/h
If your power authority charges $0.15 per Kw/h - 12Kw/h x $0.15 = $1.80 cost of charge

Using a 240V, 16A charger - 240V x 16A = 3840w or 3.84Kw. Using the 12Kw/h (what was needed to charge the car) / 3.84Kw = 3.125 hours charge time (greatly reduced)
3.84Kw x 3.125 hours x $0.15 = $1.80
Unfortunately, batteries cannot be consistently charged at a high rate until fully charged. The charging system slows down as it approaches full charge so the calculation can't be applied directly as the voltage and current are increased.

Oh yes, Kilowatt is a metric unit :D
'15 SL 40k kms, 12 bars

css28
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Re: Why do we still confuse KW and KWH?

Thu Aug 29, 2019 7:40 am

"If it uses the 1.2Kw constantly for 1 hour then the rate of usage is 1.2Kw/hr"

Absolutely not. Watts (and by extension, kilowatts) represent a rate of energy transfer (those energy units being coulombs). The total energy transferred at a rate of 1.2 kW over an hour is 1.2 kW-hrs (kW X hr, not kW/hr).

Edit: I understand that there is a convention used by some of depicting a unit that's the product of units using the "slash" symbol. I strongly recommend instead using a hyphen or a floating dot. The slash symbol (to me) represents a division symbol, leading me to read kW/hr as "kilowatts per hour" which is the source of my rant above.
- Chris
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JayCan
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Re: Why do we still confuse KW and KWH?

Thu Aug 29, 2019 8:31 am

css28 wrote:
Thu Aug 29, 2019 7:40 am
"If it uses the 1.2Kw constantly for 1 hour then the rate of usage is 1.2Kw/hr"

Absolutely not. Watts (and by extension, kilowatts) represent a rate of energy transfer (those energy units being coulombs). The total energy transferred at a rate of 1.2 kW over an hour is 1.2 kW-hrs (kW X hr, not kW/hr).
Apologies for my use of a hyphen incorrectly. I don't wish to offend with a grammatical error.
I'm simply looking at it as the power companies do when billing you. Their billing statements charge you per Kilowatt hour, however they list it, whether it be KWh, kWh. They monitor the voltage they provide, the current draw (coulombs (6.24 x 10^18 electrons) per second) you use, and the length of time you utilize both. We have different rates for time of day usage (peak or off times) so the charge per kWh changes.
The point I was wishing to make was that sometimes we use the term watt and kilowatt as a 'hard' entity, like kilogram or pound, which is confusing.
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goldbrick
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Re: Why do we still confuse KW and KWH?

Thu Aug 29, 2019 9:13 am

Since we're being precise - kilogram is a unit of mass while pound is a unit of force.

If measured with a torque wrench, 1 ft-lb (or more correctly 1 lb-ft) is the torque generated by 1 lb of force using a 1 ft bar. Its corresponding metric unit is a Newton-meter. Torque wrenches don't (or shouldn't!) use kilogram-meter units.

But then, recipes, etc call for a certain number of grams for ingredients. These values will be measured with a scale which will actually measure the force generated by the mass (Newtons) although the display will probably show the result in grams assuming 1G of gravity.

In this case, a scale showing the imperial measure of pounds/ounces etc is a more accurate representation than a metric scale showing the results in grams.

So the imperial and metric systems both are used incorrectly in many everyday scenarios. So what? If someone says they weigh 100 kilos I can understand what they mean even if the units aren't correct. As I mentioned above, virtually no one uses the imperial units for mass which is slugs. Thankfully! :mrgreen:

But then kW and kWh are quite different things and if someone wants to calculate how long it will take to charge their car or what kind of efficiency they get while driving it, a clear understanding of the difference is important. And then there are the cases like cwerdna linked to where someone is wrong and just insists on staying wrong...

css28
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Re: Why do we still confuse KW and KWH?

Thu Aug 29, 2019 9:33 am

goldbrick wrote:
Thu Aug 29, 2019 9:13 am
...In this case, a scale showing the imperial measure of pounds/ounces etc is a more accurate representation than a metric scale showing the results in grams.

So the imperial and metric systems both are used incorrectly in many everyday scenarios. So what? If someone says they weigh 100 kilos I can understand what they mean even if the units aren't correct.
They're equally accurate because they're both measuring the force resulting from the mass influenced by gravity.

If someone says they weigh 100 kilograms the units are entirely correct. Their mass is their mass. If gravity diminished, their mass would remain the same while the gravity force from their mass (in Newtons, if you want) would change.
- Chris
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2017 SV, purchased 8/10/19, Coulis red

goldbrick
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Re: Why do we still confuse KW and KWH?

Thu Aug 29, 2019 1:09 pm

We're diving into semantics here but how much does a kilogram weigh in a zero-G situation, aka 'weightlessness'?

LeftieBiker
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Re: Why do we still confuse KW and KWH?

Thu Aug 29, 2019 2:28 pm

goldbrick wrote:
Thu Aug 29, 2019 1:09 pm
We're diving into semantics here but how much does a kilogram weigh in a zero-G situation, aka 'weightlessness'?
It weighs nothing*, but still masses a kilogram.




* Genuine absence of any gravity at all is fairly rare in space.
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coulomb
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Re: Why do we still confuse KW and KWH?

Thu Aug 29, 2019 3:38 pm

goldbrick wrote:
Thu Aug 29, 2019 9:13 am
kilogram is a unit of mass while pound is a unit of force.
My understanding is that actually the pound is a measure of mass, and when colloquially used as a force, it is actually called the pound-force (unit lbf). Wikipedia: pound (force).
So the imperial and metric systems both are used incorrectly in many everyday scenarios. So what?
If we ignore for the moment the conflation of mass and weight, the metric system is (virtually?) always used correctly. I can't say the same for the imperial system, although I suspect it's possible to use it just as correctly as the metric system. I haven't used the imperial system (except when talking to North Americans and occasionally to the British) for 40 or 50 years, so I'll admit I'm no expert in the imperial system.

[ Edit: Perhaps when space travel is more common, we'll define a separate SI unit for weight, and lose one of the last inaccuracies. Edit2: I'm told that actually there already is; it's the Newton. Yes, that's the unit of force, but that's what weight in space really means: "the weight of a body in a particular reference frame is defined as the force that gives the body an acceleration equal to the local acceleration of free fall in that reference frame [4: ISO 80000-4]" (from this page; thanks Weber). ]

As for "So what?", there is a correct way to communicate things, and it should always be used in things like scientific papers, but I can't see why it should not be used in everyday life as well. If we all put a bit more effort into getting it right, I think that the newcomers to our forums (fora?) will have a better chance of getting it right, and to be less confused.

Back on topic a little (sorry ;) ). This link sets out proper spelling of SI units: How to tell a kilowatt hour from a kelvin week henry.

[ Edit: "as the imperial system" -> "as the metric system" ]
Last edited by coulomb on Sun Sep 01, 2019 5:04 am, edited 5 times in total.

goldbrick
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Re: Why do we still confuse KW and KWH?

Thu Aug 29, 2019 3:59 pm

Interesting stuff but that wikipedia link lost me when they defined the pound-force in terms of the avoirdupois pound. If you look that one up it gets pretty interesting. Note that gravity is defined in metric units for much of the definitions! Somehow Occam's razor seems applicable to much of this discussion.

I still remember calculating basic physics problems with imperial units and the unit for mass was the slug. Maybe the textbooks were just trying to show how superior metric units were 8-) In any case, I'm a convert as are almost all scientists today.

But sorry to stray so far off-topic. Language means what people understand it to mean. A rose is a rose and all that.

SageBrush
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Re: Why do we still confuse KW and KWH?

Thu Aug 29, 2019 4:30 pm

goldbrick wrote:
Thu Aug 29, 2019 3:59 pm
Interesting stuff but that wikipedia link lost me
.
I struggled too.

I *think* the issue is that four basic units exist, but only three are needed for a complete description of a an element in motion. The UK actually have two systems, to accommodate preferences.
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