Units: Kilowatt-hours vs. Megajoules

scaesare · Jan 24, 2016

[begin_snowed_in_random_musings]

Let's face it: expressing the capacity or SoC of a battery pack is awkward for the average person. "Gallons" is easy, and is clearly a measurement of capacity that's intuitive to understand.

"Kilowatt-hours" is not only a mouthful, but it's non-intuitive in that a kilowatt is a measurement of power, but must be expressed in conjunction with time in order to express energy, hence kilowatt-hour. For many folks this is rather confusing, doesn't directly relate to the application in their mind("Wait, so if I get 3 miles perkilowatt-hour, does that mean only if I'm driving at 3mph??"), and leads to incorrect usage (i.e.- saying "85 kilowatt battery pack").

So is "Megajoule" any better? It's still a mouthful, but seemingly somewhat less so (no hyphen!). It is a pretty foreign unit to most folks... but then again so is kilowatt-hour. And that may be an advantage in that folks won't try to shoehorn it in to an existing usage pattern, as they might with kilowatts.

Admittedly "300 megajoule battery pack" is not exactly snappy, but at least a bit better (3 syllables vs 5!). I also wonder if "M-joules" would catch on. There's also the chance that you'd end up with something like calories, where the vernacular "calories" when talking about food is actually a "kilocalorie" (or large calorie).

I may just start referring to my battery as a "Three hundred megajoule pack" for fun to see what happens.

mikeash · Jan 24, 2016

Kilowatt-hour is a stupid unit that sounds stupid. Megajoule is a great unit that sounds awesome. Just thinking it makes you cooler!

AudubonB · Jan 24, 2016

Well, this could get fun. Let me muddy the waters right away by complaining that "mph" or "kph" is so awkward - having to make use of not one, but two, units to demonstrate velocity. I propose we should invent a term to encompass both.

Let's call it "knots".

Cottonwood · Jan 24, 2016

This has been in my signature line on TMC for a few years:

One MegaJoule is approximately one rated mile of energy, a wonderfully obscure coincidence.

Here is the derivation:

1 W-hr = 3,600 Joules (1 W * 3,600 sec/hr = 3,600 Joules; 1 W = 1 Joule/sec)
1 MegaJoule = 1,000,000 Joules
1 MegaJoule = 1,000,000 / 3,600 = 278 W-hr

My reference is an approximate one, and 1 MJ/mi or 278 Wh/mi and the original Model S gets about 290 Wh/mi or 1.044 MJ/rated mile, but 1.044 can be rounded to 1 MJ/rated mile pretty quickly. :biggrin:

scaesare · Jan 24, 2016

mikeash said:
Kilowatt-hour is a stupid unit that sounds stupid. Megajoule is a great unit that sounds awesome. Just thinking it makes you cooler!

I like this argument!

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AudubonB said:
Well, this could get fun. Let me muddy the waters right away by complaining that "mph" or "kph" is so awkward - having to make use of not one, but two, units to demonstrate velocity. I propose we should invent a term to encompass both.

Let's call it "knots".

Just make sure it's a slightly different linear measurement depending on where you are travelling... you know, to ease confusion.

ratsbew · Jan 24, 2016

Things like this could end up being one of the biggest hurdles for EV adoption. People like us understand it, but the general population doesn't have the education or desire to understand concepts like this. Personally I think kWh is a perfectly fine unit. It makes intuitive sense to me what it means.

ArtInCT · Jan 24, 2016

Who defined the "Joule"? Gosh, I really hope that Jules Verne is in the history of the Joule.... ahhh yes, yes.., but I believe this not to be the case... no, no.

10" of new powder with drifts of 24" to 0"

Cottonwood · Jan 24, 2016

The Joule is named after James Prescott Joule.

From James Prescott Joule - Wikipedia, the free encyclopedia:

James Prescott Joule (24 December 1818 – 11 October 1889) was an English physicist and brewer, born in Salford, Lancashire. Joule studied the nature of heat, and discovered its relationship to mechanical work (see energy). This led to the law of conservation of energy, which led to the development of the first law of thermodynamics. The SI derived unit of energy, the joule, is named after James Joule. He worked with Lord Kelvin to develop the absolute scale of temperature the kelvin. Joule also made observations of magnetostriction, and he found the relationship between the current through a resistor and the heat dissipated, which is now called Joule's first law.

Jeff N · Jan 24, 2016

scaesare said:
[begin_snowed_in_random_musings]

Let's face it: expressing the capacity or SoC of a battery pack is awkward for the average person....

Admittedly "300 megajoule battery pack" is not exactly snappy, but at least a bit better (3 syllables vs 5!). I also wonder if "M-joules" would catch on. There's also the chance that you'd end up with something like calories, where the vernacular "calories" when talking about food is actually a "kilocalorie" (or large calorie).

If you want average people to relate better to the battery pack energy then why not really just use food calories when talking to people where the food labels use calories (kilocalories) as in the US?

So, a 60 kWh battery pack would be 51,600 food calories (51.6 megacalories) or about 20-25 days worth of a daily human diet.

In places like Mexico you would use megajoules since that is what their food labels use.

green1 · Jan 24, 2016

My power meter is in kwh, my charging is done in kw per hour, so kwh is the ideal measurement for this.

cpa · Jan 24, 2016

Valid points. However, most people with half a brain realize that they purchase electricity from their utility in kilowatt hours. They know that light bulbs and other household items have wattage associated with their uses. They have an understanding that the electric dryer or HVAC uses a lot more electricity than their television or laptop. Granted they cannot grasp the difference between a watt and a watt-hour, but that is the way it is with any technical subject.

I have occasionally explained that the capacity of my 85kWh battery pack could keep a 100-watt light bulb burning 850 hours, or their 5kW HVAC running for 17 hours. That generally gets the point across.

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?)

Since my average since purchase has been 285 Wh/mile, that means that I drive about 3 1/2 miles per kWh. So it is relatively easy to say that I can drive 17-18 miles using the same amount of electricity as it would take us to use our 5kW air conditioner for one full hour.

scaesare · Jan 24, 2016

cpa said:
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?)

I agree that would make more sense... I've wondered that too.

AudubonB · Jan 24, 2016

So too has wondered the entire metric world, who measure fuel consumption in liters/100km

jerry33 · Jan 24, 2016

AudubonB said:
So too has wondered the entire metric world, who measure fuel consumption in liters/100km

Probably almost everyone.

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Cottonwood said:
This has been in my signature line on TMC for a few years:

One MegaJoule is approximately one rated mile of energy, a wonderfully obscure coincidence.

Here is the derivation:

1 W-hr = 3,600 Joules (1 W * 3,600 sec/hr = 3,600 Joules; 1 W = 1 Joule/sec)

1 MegaJoule = 1,000,000 Joules

1 MegaJoule = 1,000,000 / 3,600 = 278 W-hr

My reference is an approximate one, and 1 MJ/mi or 278 Wh/mi and the original Model S gets about 290 Wh/mi or 1.044 MJ/rated mile, but 1.044 can be rounded to 1 MJ/rated mile pretty quickly. :biggrin:

The problem is that people pay in kWh. Change the metres and payment to Joules and I'll gladly support using Joules, otherwise it's another needless conversion.

ItsNotAboutTheMoney · Jan 24, 2016

jerry33 said:
Probably almost everyone.

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The problem is that people pay in kWh. Change the metres and payment to Joules and I'll gladly support using Joules, otherwise it's another needless conversion.

People also generally measure their time in hours, so kWh is useful. Onlh issue with kWh is that someone needs to force manufacturers to quote usable capacity and capacity.

Barry · Jan 24, 2016

green1 said:
My power meter is in kwh, my charging is done in kw per hour, so kwh is the ideal measurement for this.

Exactly! This is on the meter and the electric bill. it's a unit people may not understand, but they've probably seen it.

brucet999 · Jan 24, 2016

scaesare said:
[begin_snowed_in_random_musings]

Let's face it: expressing the capacity or SoC of a battery pack is awkward for the average person. "Gallons" is easy, and is clearly a measurement of capacity that's intuitive to understand.

"Kilowatt-hours" is not only a mouthful, but it's non-intuitive in that a kilowatt is a measurement of power, but must be expressed in conjunction with time in order to express energy, hence kilowatt-hour. For many folks this is rather confusing, doesn't directly relate to the application in their mind("Wait, so if I get 3 miles perkilowatt-hour, does that mean only if I'm driving at 3mph??"), and leads to incorrect usage (i.e.- saying "85 kilowatt battery pack").

So is "Megajoule" any better? It's still a mouthful, but seemingly somewhat less so (no hyphen!). It is a pretty foreign unit to most folks... but then again so is kilowatt-hour. And that may be an advantage in that folks won't try to shoehorn it in to an existing usage pattern, as they might with kilowatts.

Admittedly "300 megajoule battery pack" is not exactly snappy, but at least a bit better (3 syllables vs 5!). I also wonder if "M-joules" would catch on. There's also the chance that you'd end up with something like calories, where the vernacular "calories" when talking about food is actually a "kilocalorie" (or large calorie).

I may just start referring to my battery as a "Three hundred megajoule pack" for fun to see what happens.

Why stop with M-joules? Abbreviate it down to MJ and you could represent it with a single-glove icon.

Dutchie · Jan 24, 2016

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

International System of Units - Wikipedia, the free encyclopedia

Watt - Wikipedia, the free encyclopedia

McRat · Jan 24, 2016

Perhaps just your K number for economy. km/kWh.

"I was getting 6 K for mileage in the summer, but only 4 K this winter, but I think point five of that is the snow tires on the heavy rims."

This would equate to about 3.7 miles per kWh and 2.5 miles per kWh.

Packs aren't a big problem since you tend to abbrev in conversation: "Yeah, the battery got bumped to ninety kay-double-you this year."

green1 · Jan 25, 2016

Dutchie said:
The whole world - except the three countries mentioned - uses the correct unit for Energy which is the Joule.

Do any countries have power meters and power bills measured in Joules? I know you certainly don't living in Canada, and that's not one of the three countries you just listed. In fact, I'm not aware of anyone in Canada who ever refers to electric energy use in Joules, so I suspect you're not being 100% accurate here.

Units: Kilowatt-hours vs. Megajoules

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