Units: Kilowatt-hours vs. Megajoules

[Soolim]

Let us be practical. There is no common unit of energy that can be universally applicable for all situation, it depends on the application.
The most commonly used I know of are:
electrical energy = kWH
carbon based fuel = megaJoules
food = calories

 

[scaesare]

Let us be practical. There is no common unit of energy that can be universally applicable for all situation, it depends on the application.
The most commonly used I know of are:
electrical energy = kWH
carbon based fuel = megaJoules
food = calories

Who suggested one unit of energy be applicable to all situations? I'm talking EV's.

I understand the "commonly used" point, but to play devil's advocate:

Why is the joule (a unit of energy derived from force over distance) any more applicable to a carbon fuel source than electrical energy?

Both require conversion to mechanical energy to do work...

 

[Wilcino]

There are only really 2 reasons to choose a specific unit over another unit of the same dimensions.

1) It expresses the numbers in a reasonable amount. In the metric system this is often taken into account by a prefix. So for cars km is a good unit because people are generally talking about distances >1km and <1000km, however if we used feet, it would be pretty annoying because everything would be in tens or hundreds of thousands of feet, which is harder for people to inuitively understand. So here you'd want to talk MJ rather than J.

2) Because it allows comparison to a pre-existing common standard. Here EV cars struggle because we trying to bridge 2 pre-existing standards, the first is the electric industry, where the units of energy are kWh and power kW (W for small appliances). kWh are a good unit because when people generally come across them they're dealing in 1s to 100s. The second is the ICE car industry, where power is horsepower (same 2-syllables as megajoules!) and energy is in gallons (which not technically the correct dimensions but can be converted to them using an assumed factor, the standard calorific value of petrol / diesel). If you want to know how your car is costing you / how fast it will charge from the electricity grid, kWh is the best way to go, and I think this is the most useful unit. Granted it does confused people by being so similar to kW, which is also used, however in the event of mis-use it is generally clear from the context (although I did answer a question on here this morning where we were discussing the concept of "power density"). To compare to cars, maybe we need to use a equivalent-Gallons (eG) unit to allow comparison. However I'm British so we're just opening a whole new can of worms there.

kWh is annoying because it is easier (certainly for me) to grasp the concept of X-per-Y rather than X-times-Y. Would it work better if you have MJ and MJph?

 

[Soolim]

Who suggested one unit of energy be applicable to all situations? I'm talking EV's.

I understand the "commonly used" point, but to play devil's advocate:

Why is the joule (a unit of energy derived from force over distance) any more applicable to a carbon fuel source than electrical energy?

Both require conversion to mechanical energy to do work...

Because kWh is the unit for commerce like x cents per kWh. So how much do you pay to charge your 300 MJ car? I bet you have to convert to kWh for that cost estimate.
If Tesla advertise the car in MJ, will it be easier for most people to estimate the cost of energy for the use of the car?
You pay for electricity by kWh of consumption, and you pay for natural gas heating by MJ of consumption.
Name me one benefit of using MJ to spec the stored energy of a BEV, when compared to kWh.

BTW, is your power meter on the car to be converted to MJ/s ?:tongue:

 

[Rocky_H]

+1000. It is very painful that humanity settled on decimal. Any Alien species who was born with either 8 or 16 fingers would be far more advanced than us.

The problem with metric is it can't be reduced to base 2, which throws self-relativism out of the window, meaning you can't do natural division. It's very easy to e.g. divide a quantity of something by 8. Divide by 2, which you can use a scale and move quantity around till it balances. Then repeat 3 times. Divide by 10 on the other hand... you have no hope of doing that without using an outside measuring device. You can only get self-relativism with base 2, 4, 8, 16, 32 etc. Of those, either 8 or 16 would have worked better than 10.

I couldn’t find it in a search just now, but there was an interesting article by a guy about how even countries that say they use the metric system kind of don’t for a lot of things. Among a few examples like pint, cup, etc. he asked about how carpentry worked, since a meter couldn’t really be divided into halves, thirds, fourths easily. The person said that because of this problem, lumber was measured by a standardized measurement of 120 cm, so it could easily be divided into those fractions. So they had to create a whole different made up unit for “board lengths”, because meter doesn’t practically work for that use.

 

[ZBB]

Regarding Imperial vs SI/Metric systems and the US in particular. I just want to point out that:

1) I-19 (which is only in Arizona and runs from Nogales to Tucson) is signed in metric for distances, but MPH for speed.
2) Certain liquid beverages are sold in 2L or 750ML bottles

This is proof that the US started metrication -- but we just never finished!

 

[cpa]

Regarding Imperial vs SI/Metric systems and the US in particular. I just want to point out that:

1) I-19 (which is only in Arizona and runs from Nogales to Tucson) is signed in metric for distances, but MPH for speed.
2) Certain liquid beverages are sold in 2L or 750ML bottles

This is proof that the US started metrication -- but we just never finished!

Call me a cynic, but I really believe that the change in the bottling of our favorite beverages from fluid ounces to milliliters had more to do with selling less product at the equivalent price. For eons, distilled spirits and wines were sold in fifths--a fifth of a gallon. That equates to 25.6 fluid ounces, while 750 milliliters equates to 25.3605 fluid ounces. That is nearly a quarter fluid ounce less for the 750 ml package. Individually, that reduction is trivial to any consumer. However, when major distillers and wineries produce millions upon millions of bottles of product per year, and sell them at the same price, that tiny fraction is an instant increase to the bottom line, which may not be trivial to their financial statements.

 

[evp]

You know, the whole world adopted The International System of Units (S.I.) except the US together with Burma and Liberia (odd little group isn't it) The whole world - except the three countries mentioned - uses the correct unit for Energy which is the Joule. A Joule is the same as a Watt per second. Watt is the correct unit for power. You can also use MJ, of course, which is 1000 joules

Just to be needlessly picky: Joules are Watts * seconds, and MegaJoules are 1,000,000 Joules.

 

[ThosEM]

Just to be needlessly picky: Joules are Watts * seconds, and MegaJoules are 1,000,000 Joules.

That's not needlessly picky at all. It is correct, which cannot be said of the post about which which you commented.

I cannot believe that we have serious advocates of ridiculous mixed units here. The best units are the ones that most other people use, just as for languages. Units are arbitrary and meaningless, except in communication, for which they are essential.

 

[scaesare]

That's not needlessly picky at all. It is correct, which cannot be said of the post about which which you commented.

I cannot believe that we have serious advocates of ridiculous mixed units here. The best units are the ones that most other people use, just as for languages. Units are arbitrary and meaningless, except in communication, for which they are essential.

Who's advocating for mixed units?

 

[Soolim]

I cannot believe that we have serious advocates of ridiculous mixed units here. The best units are the ones that most other people use, just as for languages. Units are arbitrary and meaningless, except in communication, for which they are essential.

I agree that units are for common understanding therefore it is applied based on common usage. Other than the fundamental units, derived units are attributed to (in honor of) distinguished inventor whose work contributed to the science, nothing wrong with that, hence not totally arbitrary. Derived units serve the purpose of defining physical properties in a concise manner, in that sense they are meaningful.

 

[ratsbew]

As to the MPG vs wH/Mile, I have wondered why with liquid fuels we have the distance as the numerator and the quantity as the denominator, while with BEVs we invert the fraction. Would it not be more sensible to flip the BEV fraction to reflect miles per kWh (or the megajoule if so adopted?)

As a mathematically inclined person, both of these make intuitive sense to me. I think I prefer Wh/Mile because it shows the rate of use in a bit more intuitive way.

 

[GSP]

I agree that wh/mile or per kilometer is more intuitive and easier to compare energy costs. That is why the EU uses L/100km and the US EPA now lists G/100mi statistics in addition to the old mpg numbers.

While there is a good case to use MJ instead of kWh, I prefer kWh for everyday calculations. Using the same units as your energy bill is also a plus.

GSP

PS. There is a efficiency metric that I despise: MPGe. Totally useless for any purpose in my opinion.

 

[Soolim]

PS. There is a efficiency metric that I despise: MPGe. Totally useless for any purpose in my opinion.

I am not keen on the MPGe either, leaving environmental benefit of BEV aside, I guess the impetus of that unit is for comparing ICE with BEV on energy efficiency starting at the point of purchase of fuel/electricity. One gallon of gas is equivalent to 33.7 kWh, so in that sense my 70D only pacts slightly over 2 gallon of gas but can travel 240 miles (EPA). No ICE can beat that! When 150 kWh battery comes for a range of 500 miles, ICE and gas station will be seriously threaten.