Electric vehicles

[Samson]

I wonder what the equivalent would be for electric vehicles? kWh per km? Hmm.

For electric vehicles 1 kWh seems to get you about 5 km, or at least that's a round number that's plausible.

1 kWh = 3.6x10^6 joules
joules are in m^2kgs^-2
so 3.6x10^6m^2kgs^-2/10^5m if we're doing it per 100 km again.
which is 36mkgs^-2

mkgs^-2 is more conventionally referred to as Newtons. So the equivalent fuel efficiency unit for electric vehicles would be Force rather than area? With electric vehicles having an energy efficiency of about 720 N?

Sounds extremely wrong, but no more so than measuring fuel efficiency as an area. I'm not sure what the physical interpretation of that would be, equivalent to the "tube of fuel behind you" analogy.

What do you pay at the "pump"? kWh is a totally broken unit that we only use because we used to put a coin in to keep a one bar (1kW) electric heater on. We should be using joules unless we have much better reason.

 

[EnglishEdward]

I don't mind:

James Watt was Scottish

James Watt - Wikipedia

en.wikipedia.org

James Joule was English

James Prescott Joule - Wikipedia

en.wikipedia.org

 

[Mise]

I guess the real question is what are we doing with the numbers. Mostly we are just comparing cars, and for that I think the visualness of my method would help focus peoples mind on quite what we are doing when we drive somewhere.

I must admit I am struggling to imagine an infinitessimally thin 1/20th square-millimeter bar dangling out the back of my car, much less what that would look like after 50 miles.

If we frequently need to ask ourselves "How much will it cost to drive to X" l/100km is better. If we frequently need to ask ourselves "How far can I go with X money" mpg is better than l/100km but seriously should be mpl in the UK (I do not care about mixing standards in a unit if society does it).

It is much easier to imagine a litre of petrol and 100km of road, and then imagining that i'll have used about 5 or 6 litres after travelling 100km. That is much more visual to me!

(same goes for mpg)

 

[uppi]

I wonder what the equivalent would be for electric vehicles? kWh per km? Hmm.

For electric vehicles 1 kWh seems to get you about 5 km, or at least that's a round number that's plausible.

1 kWh = 3.6x10^6 joules
joules are in m^2kgs^-2
so 3.6x10^6m^2kgs^-2/10^5m if we're doing it per 100 km again.
which is 36mkgs^-2

mkgs^-2 is more conventionally referred to as Newtons. So the equivalent fuel efficiency unit for electric vehicles would be Force rather than area? With electric vehicles having an energy efficiency of about 720 N?

Sounds extremely wrong, but no more so than measuring fuel efficiency as an area. I'm not sure what the physical interpretation of that would be, equivalent to the "tube of fuel behind you" analogy.

You arrive at the same unit by considering that J = N*m. If the engines were 100% efficient (and for electric engines it is close), it is the (distance-) average amount of force you applied to your car during the trip.

 

[Samson]

According to a fairly authoritative youtube [1] the US Dept. Energy estimates that 80% of electric car charging will be done at home. Considering that there must be at least 20% of the population that do not have off road parking (in the UK it is ~33%) that has to assume that poor people will not drive.

Spoiler [1] Right at the start of Grid Issues chapter at 09:42 :

 

[EnglishEdward]

Why do you assume that a US estimate will apply to the UK ?

 

[Plains-Cow]

Why do you assume that a US estimate will apply to the UK ?

Is the UK not an American territorial state?

 

[Arwon]

According to a fairly authoritative youtube [1] the US Dept. Energy estimates that 80% of electric car charging will be done at home. Considering that there must be at least 20% of the population that do not have off road parking (in the UK it is ~33%) that has to assume that poor people will not drive.

Spoiler [1] Right at the start of Grid Issues chapter at 09:42 :

I assume that's just stating what happens now

 

[Samson]

Why do you assume that a US estimate will apply to the UK ?

It is not exactly applying, but I could only find ours and so gave it as a data point.

 

[Aiken_Drumn]

I think electric is inevitable. We are just still too early in their development to truely enjoy them.