Units: Kilowatt-hours vs. Megajoules

[McRat]

The real problem with Joules is we seldom use seconds when measuring power, we use hours. 3600 is a PITA to deal with in base 10. Our time units are not decimal, but the metric system is. 60 seconds to a minute, then 60 minutes to an hour was developed when using a compass made 60 a great number. Very easy to graph out answers on a sheet of paper. Divide a line into 10 segments with just a compass and straight edge to see the trouble. Too late to change.

 

[scaesare]

The real problem with Joules is we seldom use seconds when measuring power, we use hours. 3600 is a PITA to deal with in base 10. Our time units are not decimal, but the metric system is. 60 seconds to a minute, then 60 minutes to an hour was developed when using a compass made 60 a great number. Very easy to graph out answers on a sheet of paper. Divide a line into 10 segments with just a compass and straight edge to see the trouble. Too late to change.

I actually think it's the introduction of a unit of time in to what is thought of as a unit if capacity is the confusing issue for many folks.

Nobody discusses their cars distance capabilities in "gallons per hour"[1]. They want to know how many miles they can travel with the capacity of fuel they have. If my get's 30MPG I can drive from here to work on 1 gallon of gas. Every 30 miles I consume 1 gallon.

If I consume 387wh/mile for 30 miles, what is that in relation to my pack? (let's see here, 387x30/1000-Kwh). If it takes me 45 minutes to drive somewhere at an average of 411wh/mile, what is my usage in relation ot my pack capacity (in kilowatt-hours)?A megajoule removes time from the equation. It's a self-contained unit of energy, like gallons. As was pointed out, I can go a mile every MJ. It makes the answers to the above questions a no brainer for many folks.


[1] Although admittedly folks who run things like genrators do

 

[green1]

I actually think it's the introduction of a unit of time in to what is thought of as a unit if capacity is the confusing issue for many folks.

Nobody discusses their cars distance capabilities in "gallons per hour"[1]. They want to know how many miles they can travel with the capacity of fuel they have. If my get's 30MPG I can drive from here to work on 1 gallon of gas. Every 30 miles I consume 1 gallon.

If I consume 387wh/mile for 30 miles, what is that in relation to my pack? (let's see here, 387x30/1000-Kwh). If it takes me 45 minutes to drive somewhere at an average of 411wh/mile, what is my usage in relation ot my pack capacity (in kilowatt-hours)?A megajoule removes time from the equation. It's a self-contained unit of energy, like gallons. As was pointed out, I can go a mile every MJ. It makes the answers to the above questions a no brainer for many folks.


[1] Although admittedly folks who run things like genrators do

But would you want your gas tank capacity, and fuel gauge to be completely independent from the unit used by the pump that fills it? Imagine a car that rated it's fuel tank capacity in gallons, showed real time consumption in gallons, and was entirely in gallons. But every gas station dispensed it's fuel in litres? it would be incredibly frustrating.

As long as every utility meters in kWh it would be ridiculous for an EV company to use Joules.

 

[McRat]

I actually think it's the introduction of a unit of time in to what is thought of as a unit if capacity is the confusing issue for many folks.

Nobody discusses their cars distance capabilities in "gallons per hour"[1]. They want to know how many miles they can travel with the capacity of fuel they have. If my get's 30MPG I can drive from here to work on 1 gallon of gas. Every 30 miles I consume 1 gallon.

If I consume 387wh/mile for 30 miles, what is that in relation to my pack? (let's see here, 387x30/1000-Kwh). If it takes me 45 minutes to drive somewhere at an average of 411wh/mile, what is my usage in relation ot my pack capacity (in kilowatt-hours)?A megajoule removes time from the equation. It's a self-contained unit of energy, like gallons. As was pointed out, I can go a mile every MJ. It makes the answers to the above questions a no brainer for many folks.


[1] Although admittedly folks who run things like genrators do

We will just have to agree to disagree.

I pull into a 50kW station. I pick up 30 minutes worth. I have 25kWh. My car is getting 5 K economy today the way I'm driving. So I have 25 * 5 = 125 km of juice.

Next stop I need at least 250km worth to get home. Let's see, at 5 K that's 50kWh, this charger is 80kW. So I need 5/8ths of an hour, so just more than 1/2 an hour. I'll put 45 minutes in.

 

[jcaspar]

There is not a single country in the world that has adopted the metric system for all units, i.e. time. I think the US should hold out till then. Besides, it you have ever been to the UK, you will see they still uses Imperial units for many things (body heights and weights and of course pints of beer)

I see the Metic system as a quaint but archaic system developed in the days before calculators when dividing by 12 was just too difficult to do on a slide rule...:smile:

Forget Joules or KWhr, lets use horsepower-hours, foot-pounds or BTU's.

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

 

[Cottonwood]

Forget Joules or KWhr, lets use horsepower-hours, foot-pounds or BTU's.

Let's go for the gusto: BTU's (British Thermal Unit) — 1 BTU is the energy needed to raise the temperature of 1 pound of water 1˚ F.

1 MJ = 948 BTU's so that is 990 BTU's per rated mile.

I really like 1,000 BTU's per rated mile; that's a number that I can remember. :biggrin:



If you want to make your choice of Energy Units, there are a lot. See a list at Energy units - Wikipedia

 

[deonb]

Our time units are not decimal, but the metric system is. 60 seconds to a minute, then 60 minutes to an hour was developed when using a compass made 60 a great number. Very easy to graph out answers on a sheet of paper. Divide a line into 10 segments with just a compass and straight edge to see the trouble. Too late to change.

+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.

There is not a single country in the world that has adopted the metric system for all units, i.e. time. I think the US should hold out till then.

Seeing that the official metric unit for time is seconds, technically every country in the world has adopted it. It's not a particularly great unit, but it is standard metric. Like Celcius as well - it really has no mathematical benefits over Fahrenheit, but it's standard.



Coming back to the OP. Joules isn't quite the correct unit for a battery charge either. A more correct unit would be Coulomb.

 

[Cottonwood]

Coming back to the OP. Joules isn't quite the correct unit for a battery charge either. A more correct unit would be Coulomb.

I beg to differ here. We are interested in the energy stored in the battery, not the total charge accumulated.

BTW: 1 Coulomb * 1 Volt = 1 Joule (1 Volt = 1 Joule/Coulomb)

 

[scaesare]

But would you want your gas tank capacity, and fuel gauge to be completely independent from the unit used by the pump that fills it? Imagine a car that rated it's fuel tank capacity in gallons, showed real time consumption in gallons, and was entirely in gallons. But every gas station dispensed it's fuel in litres? it would be incredibly frustrating.

As long as every utility meters in kWh it would be ridiculous for an EV company to use Joules.

No, I'd suggest all the display's matching would be most useful. And incidentally the "pump" that fills the car, the Supercharger, has no such display.. it's all in-car.

If you think about it, the rating of a supercharger is a unit of power (KW), not energy (KWh)... which is much more analogous to the pressure or flow rate of a gas pump... who uses that?

Make the center screen count MJ's "dispensed", just as a gas pump counts gallons dispensed.

Easy.

 

[green1]

No, I'd suggest all the display's matching would be most useful. And incidentally the "pump" that fills the car, the Supercharger, has no such display.. it's all in-car.

If you think about it, the rating of a supercharger is a unit of power (KW), not energy (KWh)... which is much more analogous to the pressure or flow rate of a gas pump... who uses that?

Make the center screen count MJ's "dispensed", just as a gas pump counts gallons dispensed.

Easy.

there's no display because there's no billing.
At home (where most of my charging is done) there is a display, and a bill, and both are in kWh

 

[scaesare]

there's no display because there's no billing.
At home (where most of my charging is done) there is a display, and a bill, and both are in kWh

Yup, but you asked why you'd want a different unit than the "pump that fills it".... that's my point: There are no units on a Supercharger, unless you go snooping and look at the plates on the cabinets behind the fence, the average person has no idea what they are capable of... only if they are the "new cool ones with lighted letters". There's no marking on the cabinets.

We only know here because we're nerds.

What's more, the Superchargers and pack are NOT rated with the same units: the supercharger is rated in kilowatts (a unit of power), whereas the pack is kilowatt-hours (a unit of energy).

 

[green1]

Yup, but you asked why you'd want a different unit than the "pump that fills it".... that's my point: There are no units on a Supercharger, unless you go snooping and look at the plates on the cabinets behind the fence, the average person has no idea what they are capable of... only if they are the "new cool ones with lighted letters". There's no marking on the cabinets.

We only know here because we're nerds.

What's more, the Superchargers and pack are NOT rated with the same units: the supercharger is rated in kilowatts (a unit of power), whereas the pack is kilowatt-hours (a unit of energy).

It's not about "rating" it's about what they are charged for.

EVERYWHERE that EVER charges you for the electricity you put in your vehicle charges in kWh. EVERY SINGLE PLACE. you would very definitely not want the car dealing in anything else.

So your complaint needs to start with the power utilities, not Tesla.

 

[Dutchie]

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.

Actually I do live in Canada, and Canada is odd in respect to S.I. They formally adopted it but - being so close to the US - we still use foots, pounds etc. But we do use kilometres, litres etc. I use to live in the Netherlands and I believe my power bill there was indeed in MJoule.

 

[green1]

Actually I do live in Canada, and Canada is odd in respect to S.I. They formally adopted it but - being so close to the US - we still use foots, pounds etc. But we do use kilometres, litres etc.

So your "three countries" has now become four, and I bet it's going to become a whole lot more before we're done.

I just looked up the UK, they bill in kWh, same for the Netherlands (despite what you just said) and in fact, it seems the whole EU uses kWh.
From what I can find, Australia also kWh, as is Japan.

In fact, from my google searches I have yet to find ANY country where people get billed in Joules.

Anyone on here want to post that they receive their power bill in Joules? This is an international forum, should be easy to find someone who pays in Joules if your thought that nobody outside the US uses kWh is true.

 

[Dutchie]

There is not a single country in the world that has adopted the metric system for all units, i.e. time. I think the US should hold out till then. Besides, it you have ever been to the UK, you will see they still uses Imperial units for many things (body heights and weights and of course pints of beer)

I see the Metic system as a quaint but archaic system developed in the days before calculators when dividing by 12 was just too difficult to do on a slide rule...:smile:

Forget Joules or KWhr, lets use horsepower-hours, foot-pounds or BTU's.

Well, th UK and Canada are the only countries who still use both. Canada, because of the influence of the US and the UK because..???

- - - Updated - - -

So your "three countries" has now become four, and I bet it's going to become a whole lot more before we're done.

Anyone on here want to post that they receive their power bill in Joules? This is an international forum, should be easy to find someone who pays in Joules if your thought that nobody outside the US uses kWh is true.

no, it officially has accepted SI, but in reality it uses both. Same with UK. All other countries uses SI.

 

[green1]

no, it officially has accepted SI, but in reality it uses both. Same with UK. All other countries uses SI.

What system a country claims to use is irrelevant for this purpose. The point is that NOBODY pays for electricity in Joules, EVERYONE uses kWh. So why on earth would you ever want to use Joules in your electrically powered device if it doesn't translate to how you buy your power?

See my edits above. I have been unable to find any country on the entire planet that bills in Joules. it's all kWh

 

[Dutchie]

What system a country claims to use is irrelevant for this purpose. The point is that NOBODY pays for electricity in Joules, EVERYONE uses kWh. So why on earth would you ever want to use Joules in your electrically powered device if it doesn't translate to how you buy your power?

See my edits above. I have been unable to find any country on the entire planet that bills in Joules. it's all kWh

Well, I looked it up. In the Netherlands we also us kWh. It is not an S.I. unit, however in this particular case it is accepted by the International Bureau of Weights and Measures.
see Kilowatt hour - Wikipedia, the free encyclopedia

 

[grichard]

Pirate-ninjas.

Just sayin'.

 

[scaesare]

It's not about "rating" it's about what they are charged for.

EVERYWHERE that EVER charges you for the electricity you put in your vehicle charges in kWh. EVERY SINGLE PLACE. you would very definitely not want the car dealing in anything else.

So your complaint needs to start with the power utilities, not Tesla.

I'm not "blaming" anybody. I'm bringing up something for general discussion, even if I don't have any delusions it's going to change.

So in that context, I'd be good with utility companies changing too. :wink:

 

[AudubonB]

For how the meters read in the UK I'll have to email my nephew Jules.

<not a joke...but I finally got a chance to use this serendipity>