Why do we still confuse KW and KWH?

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cwerdna said:
smkettner said:
No one is actually confused.
...
Simple slip of the keyboard makes way too big a deal of it. For those that are always careful and deliberate to get this right perfectly every time I commend you. For the rest... let it go.
Sometimes, it's not a slip of the keyboard. Sometimes, a person keeps getting it wrong over and over. This guy is an example: http://mynissanleaf.com/viewtopic.php?p=389319#p389319. He was not just making typos. He didn't know what he was talking about. Read his further replies in that thread like
" A 40w light bulb is not 40w (that would mean it burns out after 1hr) its a 40 wh light bulb"
" A 3000w genny is actually a 3000wh genny"
"a 40w bulb is not 40watts. that would mean the bulb can only work for 1 hour. your average "cheap" incandescent 40w bulb is actually a 60,000w bulb (40watts times estimated lifespan of 1500 hours)

the RATE it consumes power is 40 watts per hour." UGH!

How about Reeler that I referred to at https://www.mynissanleaf.com/viewtopic.php?p=564743#p564743? What's surprising and scary is his background and "experience" with EVs: https://www.mynissanleaf.com/viewtopic.php?p=565245#p565245.

There are a few folks who fairly often get it wrong, say 50% of the time, not 80 to 100% of the time. For them, I think it's a lack of understanding and not a slip up either.
Yes the out of context corrections make a mess of things. I contend neither was confused.
 
smkettner said:
Yes the out of context corrections make a mess of things. I contend neither was confused.
Whatever.

How about this: take the time to try and write clearly, or if needed, take the time to LEARN. Otherwise do not expect people to take the time to answer.
 
alozzy said:
There are so many misconceptions and misunderstandings about all kinds of things, so confusion over kW vs kWh is relatively forgivable in my opinion.

I'm a lot more worried that some people still think that the earth is flat, or that there is zero gravity in space, or that the sun is actually yellow, or that science is a collection of irrefutable facts and so any "mistakes" that past scientists made, which are later refuted, somehow utterly disproves all scientific understanding and achievement (detractors of the theory of evolution love that argument).

Unfortunately but not surprisingly, the people who most aggressively think they should be able to write incorrect units without comment are also the most scientifically ignorant and much more often than not, AGW denialists.
 
EVDRIVER said:
Let's not even start on 110/220V.

Bit of a tangent, I noticed recently that our home voltage was gradually dropping over a week or so (reading from a small UPS). We're normally close to 125V. I ruled out internal wiring and figured something with the transformer or substation was overstressed from warm weather demands.

When it got down to 110V I contacted the utility. I was surprised but they actually came and checked. They confirmed that it was upstream somewhere and needed further investigation. Next day, unplanned 3000 home outage that they had to dispatch a crew. But afterwards we were back over 120V.
 
Line voltage certainly varies with demand until it hits a brownout (low voltage) or blackout. One thing that is dependable is that the frequency is always very close to 60.0 Hz. I'd bet there are regulations on both the allowable voltage level range and frequency range but the frequency is controlled to a much tighter spec.

With the advent of smart meters I wonder if the system will become more closed loop. Ie, will the various meters report feed back the voltage at that point so the network/grid can make adjustments to even out voltage variations. AFAIK, without smart meters the grid is very open-loop with the only feedback/controls at the substation levels. Or maybe there are monitoring stations placed around the grid that report back somehow.
 
goldbrick said:
Line voltage certainly varies with demand until it hits a brownout (low voltage) or blackout. One thing that is dependable is that the frequency is always very close to 60.0 Hz. I'd bet there are regulations on both the allowable voltage level range and frequency range but the frequency is controlled to a much tighter spec.

With the advent of smart meters I wonder if the system will become more closed loop. Ie, will the various meters report feed back the voltage at that point so the network/grid can make adjustments to even out voltage variations. AFAIK, without smart meters the grid is very open-loop with the only feedback/controls at the substation levels. Or maybe there are monitoring stations placed around the grid that report back somehow.

I suspect they do have automated alerts, but the voltage was still within allowable limits. What probably wasn't being watched was the trend. After seeing it creep down day after day I was like "****'s about to go down, dawg". :lol:
 
Common N. American voltages that I've seen:
120v- can sometimes drop down a bit but the term 110v is really outdated AFAIK.
208v-Commercial 3 phase 240v, can drop down close to 200v. Uses (2) 120v commercial legs.
220v-like 110v it's an outdated term AFAIK.
240v-generally residential power using (2) 120v legs.
277v-commercial 3 phase power using just 1 hot, not really used for EVs except some Teslas.
480v-commercial 3 phase power using (2) 277v legs, probably only used for QC DC stations but in this case they'd probably use all (3) 277v hots.

I believe some locations use lesser voltages than 480v for internal power and in that case I don't know if they use less than 277v/hot, I've only seen 277/480v in my line of work, I'm sure true electricians have see more types than me.

And AFA the whole KW and kWh, I'm personally one that gets confused on it frequently and often refer to old posts to get it right, same as the spelling for their, there, they're, I've been told many times what is correct and even looked it up but for some reason it's really hard for me to get it correct :? but I try :)
 
Nubo said:
EVDRIVER said:
Let's not even start on 110/220V.

Bit of a tangent, I noticed recently that our home voltage was gradually dropping over a week or so (reading from a small UPS). We're normally close to 125V. I ruled out internal wiring and figured something with the transformer or substation was overstressed from warm weather demands.

When it got down to 110V I contacted the utility. I was surprised but they actually came and checked. They confirmed that it was upstream somewhere and needed further investigation. Next day, unplanned 3000 home outage that they had to dispatch a crew. But afterwards we were back over 120V.

Power authorities have a 10% plus/minus tolerance for output supply. At 110v still acceptable but barely. Summer is the worst time for voltage drop issues due to heavy AC loads. Since there was a power outage shortly after probably one transformer of a 3 phase array in a substation yard was degraded and causing the drop in voltage. It probably tanked, dropping out that part of the grid.
 
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
 
"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.
 
css28 said:
"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.
 
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...
 
goldbrick said:
...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.
 
goldbrick said:
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.
 
goldbrick said:
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" ]
 
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 :cool: 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.
 
goldbrick said:
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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