NavyCuda
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Energy Economy displace change for metric.

Fri Jul 10, 2015 12:29 am

Currently my 2015 Leaf displays average energy economy as kWh/Km. Here in Canada at least, the standard for measuring fuel economy is L/100km.

I think that the Leaf should display the metric energy economy as kWh/100km. This will allow easier comparisons to be made in regards to energy usage. It will also follow a standard pattern and be easier for the layman to understand. Here is an example from my own vehicles:

Maxima gets ~9.5L/100km.
300zx gets ~16.5L/100km
Leaf gets ~14.9kWh/100km.

One liter of gasoline contains the equivalent of 8.9kWh.

So the Maxima takes ~84.6kWh to go 100km. The 300zx takes ~146.9kWh to travel 100km.

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dgpcolorado
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Re: Energy Economy displace change for metric.

Fri Jul 10, 2015 2:06 pm

While I see your point, I find the unit "kWh/100km" a bit kludgy; my preferred metric for EVs is Wh/km, which seems cleaner. If I want to go 135 km and I use 140 Wh/km it is easy to just multiply 140 Wh x 135 km and get 18.9 kWh. [However, the actual amount of electricity used from "the wall" would be somewhat higher because of efficiency losses while charging (level 2 charging at 6 kW should be about 90% efficient in a LEAF). But that's not what one cares about when trying to decide how far one can go on a charge.]

Anyway, comparing EV energy use to the energy equivalent of gasoline seems a bit misleading. The energy content of gasoline expressed in kWh does show how inefficient an ICE is but isn't that well known? And how do you express the efficiency losses of an EV? Charging at 6 kW is perhaps 90% efficient, unless one is preheating, when it could be considerably lower. Charging at 1.4 kW is about 75% efficient. What sort of number do you use for your kWh/100km figure? An EV with a battery Temperature Management System (TMS) has additional efficiency losses, but how could one put a number on them? The point I am trying to make is that a dash number of "kWh/100km" is not really a fair or useful comparison to L/100km, converted to kWh/100km, for an ICE car.

For EVs at this early stage the most important metric is range. Once all EVs have a long range then relative efficiency will become a more interesting number.
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abasile
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Re: Energy Economy displace change for metric.

Fri Jul 10, 2015 2:28 pm

dgpcolorado wrote:I find the unit "kWh/100km" a bit kludgy; my preferred metric for EVs is Wh/km, which seems cleaner.

I agree. The only reason, I think, for the '100km' in L/100km is that L/km would end up being a small decimal number like 0.095 L/km. Wh/km is cleaner all around.

BTW, as one with science training, I am in favor of the US switching to metric units, though I acknowledge some caveats. For instance, the Fahrenheit scale seems more intuitive than Celsius for day-to-day use because 0 is "really cold" and 100 is "really hot", and negatives aren't all that common for most of the US population.
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ElectricEddy
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Re: Energy Economy displace change for metric.

Fri Jul 10, 2015 5:14 pm

abasile wrote:
dgpcolorado wrote:I find the unit "kWh/100km" a bit kludgy; my preferred metric for EVs is Wh/km, which seems cleaner.

I agree. The only reason, I think, for the '100km' in L/100km is that L/km would end up being a small decimal number like 0.095 L/km. Wh/km is cleaner all around.

BTW, as one with science training, I am in favor of the US switching to metric units, though I acknowledge some caveats. For instance, the Fahrenheit scale seems more intuitive than Celsius for day-to-day use because 0 is "really cold" and 100 is "really hot", and negatives aren't all that common for most of the US population.
km/kWh works great because there are more km's / kWh than miles thus closer accuracy . However when it comes to temperature, I use Fahrenheit as a shift of 1*C represents a difference of 1.8*F and that is more accurate . Particularity when checking the differential on say a thermostat.
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NavyCuda
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Re: Energy Economy displace change for metric.

Fri Jul 10, 2015 6:31 pm

dgpcolorado wrote:While I see your point, I find the unit "kWh/100km" a bit kludgy...


As a general rule of thumb, the larger your sample size the more accurate the estimate. So kWh per 100km gives a better idea of how much energy it will require to take a trip. Wh/KM could be misleading because where was the sample taken?

dgpcolorado wrote:Anyway, comparing EV energy use to the energy equivalent of gasoline seems a bit misleading. The energy content of gasoline expressed in kWh does show how inefficient an ICE is but isn't that well known? And how do you express the efficiency losses of an EV? Charging at 6 kW is perhaps 90% efficient, unless one is preheating, when it could be considerably lower. Charging at 1.4 kW is about 75% efficient. What sort of number do you use for your kWh/100km figure? An EV with a battery Temperature Management System (TMS) has additional efficiency losses, but how could one put a number on them? The point I am trying to make is that a dash number of "kWh/100km" is not really a fair or useful comparison to L/100km, converted to kWh/100km, for an ICE car.


There is nothing misleading about comparing kilowatt hours to gasoline, both units can be expressed accurately as BTU's which means the conversion is straight across and accurate. While my numbers were quickly generated by the guess-oh-meter and the fuel economy for my cars was hand calculated it is sufficient to illustrate the vast difference in energy efficiency between the electric car and the internal combustion engine. I did not including charging losses as part of my calculation, though correcting for losses would not effect the final values dramatically. The Leaf is still the clear winner in energy efficiency.

dgpcolorado wrote:For EVs at this early stage the most important metric is range. Once all EVs have a long range then relative efficiency will become a more interesting number.


Range is not a metric of efficiency, it is the capability of the vehicle and is beyond the scope of this discussion. I'm talking about improving the way that energy efficiency is displayed to the driver.

abasile wrote:I agree. The only reason, I think, for the '100km' in L/100km is that L/km would end up being a small decimal number like 0.095 L/km. Wh/km is cleaner all around.


As mentioned above, the reason for unit of energy per 100km is that a larger sample size has greater accuracy.

abasile wrote:BTW, as one with science training, I am in favor of the US switching to metric units, though I acknowledge some caveats. For instance, the Fahrenheit scale seems more intuitive than Celsius for day-to-day use because 0 is "really cold" and 100 is "really hot", and negatives aren't all that common for most of the US population.


If you truely have science training and you feel the Fahrenheit scale is more intuitive, I suggest you go back for more training.

There is nothing more intuitive than using water as reference. Water at sea level boils at 100C and freezes at 0C. How is that not intuitive?

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abasile
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Re: Energy Economy displace change for metric.

Fri Jul 10, 2015 11:34 pm

NavyCuda wrote:As mentioned above, the reason for unit of energy per 100km is that a larger sample size has greater accuracy.

While I understand what you are saying, one's choice of units doesn't necessarily imply a given sample size.

NavyCuda wrote:If you truely have science training and you feel the Fahrenheit scale is more intuitive, I suggest you go back for more training.

There is nothing more intuitive than using water as reference. Water at sea level boils at 100C and freezes at 0C. How is that not intuitive?

For science, I think the Kelvin scale is the most intuitive because zero is absolute zero.

While I am personally fine with Celsius for day to day use, I was simply observing that for the general population, for non-scientific purposes, Celsius can be a bit less intuitive than Fahrenheit. With Celsius, we are more likely to have to deal with negative numbers which are less intuitive to many people. And Celsius has a smaller range of values that are relevant to what most people need on a daily basis. On the other hand, if one has grown up with Celsius, then perhaps Fahrenheit seems foreign and odd.

Regardless, I'd still like to see a nationwide switch to SI units. Maybe in another generation...
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NavyCuda
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Re: Energy Economy displace change for metric.

Sat Jul 11, 2015 12:53 am

abasile wrote:For science, I think the Kelvin scale is the most intuitive because zero is absolute zero.


For science, or any other field where the negative numbers don't work properly in the calculations. Absolute zero is not a good reference point for the average temperatures humans see in their lifetime.

abasile wrote:While I am personally fine with Celsius for day to day use, I was simply observing that for the general population, for non-scientific purposes, Celsius can be a bit less intuitive than Fahrenheit. With Celsius, we are more likely to have to deal with negative numbers which are less intuitive to many people. And Celsius has a smaller range of values that are relevant to what most people need on a daily basis. On the other hand, if one has grown up with Celsius, then perhaps Fahrenheit seems foreign and odd.

Regardless, I'd still like to see a nationwide switch to SI units. Maybe in another generation...


I grew up with both systems however I found that Celcius makes more sense. To cover the same ground again, negative numbers are easy to deal with because anything 0 or below is the freezing point of water. I can't understand how 32F is more intuitive than 0C or 212F to 100C. I would appreciate an explanation of how two numbers with no tangible reference point is more intuitive.

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abasile
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Re: Energy Economy displace change for metric.

Sat Jul 11, 2015 8:43 am

NavyCuda wrote:I grew up with both systems however I found that Celcius makes more sense. To cover the same ground again, negative numbers are easy to deal with because anything 0 or below is the freezing point of water. I can't understand how 32F is more intuitive than 0C or 212F to 100C. I would appreciate an explanation of how two numbers with no tangible reference point is more intuitive.

Scientifically, yes, Celsius makes more sense. The primary issue I see with Celsius for many people is the frequent use of negative numbers. But I suppose it's a matter of what you're used to. People come up with their own reference points. 70°F is commonly thought of as a comfortable temperature, for instance. Likewise, 20°C is a common home thermostat setting. Sure, 0°C is intuitive, but 100°C is a bit less tangible for those of us who live above sea level; where I live, water boils at closer to 94°C.
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dgpcolorado
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Re: Energy Economy displace change for metric.

Sat Jul 11, 2015 9:50 am

NavyCuda wrote:As a general rule of thumb, the larger your sample size the more accurate the estimate. So kWh per 100km gives a better idea of how much energy it will require to take a trip. Wh/KM could be misleading because where was the sample taken?
Oh come now, Wh/km can be an average over however much distance one chooses. On a dash meter it could be accumulated from the last reset. The difference between figures for kWh/100km and Wh/km is just decimal point shifting. They measure the same thing.
...Range is not a metric of efficiency, it is the capability of the vehicle and is beyond the scope of this discussion. I'm talking about improving the way that energy efficiency is displayed to the driver.
I misunderstood. I thought you were talking about marketing figures or the numbers displayed on a new car window sticker. In which case range is currently the critical number for EVs, not efficiency.

Using kWh/100km as a dash reading really doesn't make much sense to me as an improvement over Wh/km. I stand by my opinion that it is a kludgy unit and unnecessary.

abasile wrote:...Sure, 0°C is intuitive, but 100°C is a bit less tangible for those of us who live above sea level; where I live, water boils at closer to 94°C.
And water boils at 92º at my house. Since tea is supposed to be brewed at 95º, from what I've read, many years ago I started making mine by boiling the tea in the kettle, since that's the hottest I can get it here and pouring into a cup immediately lowers the temperature signficantly.

I'd love to see a switch to SI units in the USA but after the failed effort in the '70s I can't see it ever happening. Too many people too used to the old ways to ever get them to change. I commend Canada for getting it right.
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