Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register

Quick Mileage Cost Calculation, take 2

This site may earn commission on affiliate links.
do you trust the dashboard data's accuracy?

Depends on what you are referring to. The rated miles (the miles next to the gauge) click off at 230Wh/rmi (or maybe 234Wh/rmi, but I've only ever measured 230Wh/rmi). That is referenced to the trip meter gauge.

However, for recharging those miles, it takes 245Wh per rmi added back (as indicated on the charging screen). And for you with your alleged 94% efficiency, that means 260Wh/rmi from the wall.

In other words, the trip meter reads (perhaps) 4.7% low (234/245) . However there are a lot of moving pieces here, so it's hard to say what is "low" and what is "high".

There is an additional complication that the 310 rmi on your dashboard at full charge aren't really the same as the EPA 310 miles - because you have a reserve energy of a few kWh below 0%/0rmi. So actually each rated mile on the dashboard is more like 0.96 * EPA rated mile (it's a ratio of something like 76/79 or 76/78). So if you never want to go below 0rmi/0% (a good plan), you really only can go 298 EPA rated miles (0.96EPArmi/rmi * 310rmi) at the EPA efficiency. If you want to drive until the car stops, it is possible you could go another 10 or so EPA miles beyond that. But I wouldn't recommend it and you'll probably get stuck. But that's what they do in the EPA test. But they're on a dyno right next to a wall outlet, so it is cool for them.

Anyway, for your cost purposes, all you can really do is what you are doing and measure from the wall. That is accurate - as long as you measure the actual voltage and current directly after your meter (not at the car - though you might want to do that as well, to separate out the various sources of inefficiency).

Meant to quote this above:

https://iaspub.epa.gov/otaqpub/display_file.jsp?docid=46585&flag=1
 
Last edited:
Quoted the wrong number here...this was the AWD, so it is 29kWh/100mi

I double checked this. The EPA number to get the 29kWh/100mi number was 88.5% efficiency (79.2kWh available energy, 89.6kWh recharge event). 79.2/89.6 = 88.4% 89.6kWh/310 = 28.9kWh/100mi

So that explains why the numbers @ahd13 are a little better than I predicted - because his charging efficiency is quite a bit better.

I think it probably really is a bit better when charging at 48A - not clear what the rate of charge was for the EPA test. I always assumed it was done at the maximum rate for the vehicle, but maybe they used 32A which would drop the efficiency a couple % (I'm too lazy to do the math on the impact of overhead at different charge rates right now - it's fairly straightforward but have to separate the overhead from the AC/DC converter losses). You can probably estimate from the prior provided plots or @darth_vad3r 's model fit. But the plots don't go to 48A.

If you use the 94% apparent efficiency (which needs investigation - it might not be that good if you have 248V or something to your house even under load):

79.2kWh / 0.94 / 310 mi = 27.2kWh/100mi

27.2kWh/100mi * $0.23/kWh = 6.2 cents/mi

Doesn't completely explain it, since your efficiency was actually slightly below the EPA estimates (looks like your wife got maybe 250Wh/mi (indicated), but it depends on how much vampire drain you had in that 10 hours - it might have been slightly better indicated). But not by much, and effectively that is what you got (230Wh/rmi * 133rmi/122mi). Anyway, to get the EPA estimates, you need to get about 230Wh/mi * 79/76 = 240Wh/mi indicated. Have to ratio up the trip meter by the ratio of full battery capacity to capacity minus alleged reserve. Just the confusing way it works...lots of silly numbers and factors for reasons best known to Tesla. To some extent this is theoretical - someone would have to check a 100% to 0% discharge...
That's a significant amount of complexity, though to fully digest and even implement into a new test will require more time than I have now. Still important learnings - all the responses have taught me a lot, so thanks...
 
Makes sense. Question - do you trust the dashboard data's accuracy? I was trying to run my calculations without trusting it (hence odometer miles, which I do take on faith, and power measured from the wall rather than the car. I ask because it seems that even under the most optimal driving conditions and behavior, our actual range never seems to measure up to the theoretical range.

I trust that it is trying to measure something accurately, but we don’t know how hard it tries and for sure what it is measuring :)

I believe it is counting coulombs on the DC kWh “out” side of the battery. For your cost purposes you would need to back out battery in/out loss and charging loss. Accurately measuring actual wall kWh used is a better approach.

I still find it weird your end SoC is so varied.

Are you seeing these varied numbers immediately after charge ends (the miles number reported in the app “charge complete” notification)? Or the next morning when you look at the dash or check the app?
 
That's a significant amount of complexity, though to fully digest and even implement into a new test will require more time than I have now. Still important learnings - all the responses have taught me a lot, so thanks...

In the end you are doing the right thing. Measure from the wall. That's what matters, and all that really matters. The rest just helps you make sense of how those rated miles tick off and how you actually have to perform on the trip meter to get the rated miles.

But if you want to "scale" from the indicated efficiency in the car, the summary is:

1) The trip meter is useless for cost calculations since it doesn't include vampire.

2) If you still want to use it, it looks like for you, the scaling for cost, using your 94% (alleged) efficiency is about:
x Trip Meter Wh/mi * 260WWh/rmi / 230TMWh/rmi = x * 1.13 Wall Wh / mi (So multiply trip meter average by 1.13 and the miles traveled to get your total wall cost)

WWh is Wall Wh, TMWh is Trip Meter Wh. I just make up new units all the time. We also have a "convention" of iWh and oWh which I also like. iWh = TMWh.

So, you can set a lifetime trip meter and use that. However, as detailed, this calculation doesn't include vampire, so maybe add 10% to the result. It's very hard to actually get a good count of vampire drain and it depends a lot on your usage patterns (if you use Sentry mode it could be a LOT higher - this isn't vampire, it's "feature drain"). But 10% in California is a good guess if you drive 10k miles per year and don't use any features. And if Telsa's improvements hold together over the winter, it's possible it could end up lower, annualized.


Coming back to how you have to "do" to get the rated miles, the summary is, you have to get a reading on the trip meter (for a single continuous trip, not including any stops):

230Wh/rmi * 79/76 = 239Wh/rmi. Or maybe 234Wh/rmi * 79/76 = 243Wh/rmi. (I've only been able to verify the 230Wh/rmi number myself...). It may also be 78/76 (that's what it is for the LR RWD...but they might allow more discharge or higher state of charge for the LR AWD, to boost the range).

But it is safe to say that if you get 239Wh/rmi you'll be able to travel 310 miles (actually, 298 miles to 0%). And as detailed, you have to drive about 12 rated miles past 298 miles to get your full 310 miles (not recommended!). This is assuming that your battery shows 310rmi at 100%. If it doesn't, whatever your battery shows, you have to subtract that "degradation" from the above numbers. It obviously also assumes that reserve actually exists - would not count on it, but Elon says a few miles exist, FWIW.

To make it really SIMPLE, to get away from EPA numbers: To get rated mile per mile "tickoff" on the battery gauge, you need to get 230Wh/mi on the trip meter. If you are higher, you'll use rated miles at a higher rate than miles traveled. That is what you will find, consistently.
 
Last edited:
  • Like
Reactions: ahd13
I trust that it is trying to measure something accurately, but we don’t know how hard it tries and for sure what it is measuring :)

I believe it is counting coulombs on the DC kWh “out” side of the battery. For your cost purposes you would need to back out battery in/out loss and charging loss. Accurately measuring actual wall kWh used is a better approach.

I still find it weird your end SoC is so varied.

Are you seeing these varied numbers immediately after charge ends (the miles number reported in the app “charge complete” notification)? Or the next morning when you look at the dash or check the app?

Just to clarify here ... it’s only measuring (or reporting measurements to us anyways) while the car is in D,N,R (including while motionless), but not including while in P or shut off and still draining some amount of power for whatever (sentry mode or just maintaining the 12V battery).
 
I trust that it is trying to measure something accurately, but we don’t know how hard it tries and for sure what it is measuring :)

I believe it is counting coulombs on the DC kWh “out” side of the battery. For your cost purposes you would need to back out battery in/out loss and charging loss. Accurately measuring actual wall kWh used is a better approach.

I still find it weird your end SoC is so varied.

Are you seeing these varied numbers immediately after charge ends (the miles number reported in the app “charge complete” notification)? Or the next morning when you look at the dash or check the app?
Immediately or close to it. I charge at 4:00am, so this morning I actually saw it complete the charge real time. I assumed that everybody had this variation, but it looks like I need to investigate...
 
11.5 kW x 165 min / (60 min/hr) = 31.6 kWh

Where did 30.3 kWh come from?

Good point. Didn't check that multiplication while I was walking the dogs today. I just assumed it was correct. That modifies all the numbers above a bit....

133/123 * 30.3kWh*$0.23 =$7.53.

133/123 * 31.6kWh * $0.23/kWh = $7.85

So 6.4 cents/mi

Efficiency number is still 94/95% though I think. I was calculating that not using the 30.3kWh number.

All has to be compared against the actual voltage the car is getting though to calculate actual efficiency, just a rough estimate here.
 
Last edited:
In the end you are doing the right thing. Measure from the wall. That's what matters, and all that really matters. The rest just helps you make sense of how those rated miles tick off and how you actually have to perform on the trip meter to get the rated miles.

But if you want to "scale" from the indicated efficiency in the car, the summary is:

1) The trip meter is useless for cost calculations since it doesn't include vampire.

2) If you still want to use it, it looks like for you, the scaling for cost, using your 94% (alleged) efficiency is about:
x Trip Meter Wh/mi * 260WWh/rmi / 230TMWh/rmi = x * 1.13 Wall Wh / mi (So multiply trip meter average by 1.13 and the miles traveled to get your total wall cost)

WWh is Wall Wh, TMWh is Trip Meter Wh. I just make up new units all the time. We also have a "convention" of iWh and oWh which I also like. iWh = TMWh.

So, you can set a lifetime trip meter and use that. However, as detailed, this calculation doesn't include vampire, so maybe add 10% to the result. It's very hard to actually get a good count of vampire drain and it depends a lot on your usage patterns (if you use Sentry mode it could be a LOT higher - this isn't vampire, it's "feature drain"). But 10% in California is a good guess if you drive 10k miles per year and don't use any features. And if Telsa's improvements hold together over the winter, it's possible it could end up lower, annualized.


Coming back to how you have to "do" to get the rated miles, the summary is, you have to get a reading on the trip meter (for a single continuous trip, not including any stops):

230Wh/rmi * 79/76 = 239Wh/rmi. Or maybe 234Wh/rmi * 79/76 = 243Wh/rmi. (I've only been able to verify the 230Wh/rmi number myself...). It may also be 78/76 (that's what it is for the LR RWD...but they might allow more discharge or higher state of charge for the LR AWD, to boost the range).

But it is safe to say that if you get 239Wh/rmi you'll be able to travel 310 miles (actually, 298 miles to 0%). And as detailed, you have to drive about 12 rated miles past 298 miles to get your full 310 miles (not recommended!). This is assuming that your battery shows 310rmi at 100%. If it doesn't, whatever your battery shows, you have to subtract that "degradation" from the above numbers. It obviously also assumes that reserve actually exists - would not count on it, but Elon says a few miles exist, FWIW.

To make it really SIMPLE, to get away from EPA numbers: To get rated mile per mile "tickoff" on the battery gauge, you need to get 230Wh/mi on the trip meter. If you are higher, you'll use rated miles at a higher rate than miles traveled. That is what you will find, consistently.
Cool thanks for laying it out. Should be able to apply that.
 
Immediately or close to it. I charge at 4:00am, so this morning I actually saw it complete the charge real time. I assumed that everybody had this variation, but it looks like I need to investigate...

My end km for the same charge limit is usually within 1-2 km, often identical. That would translate to ~1 mile only on the dash.

It’s also worth noting that after an hour of sitting (or less) you might see a shift in the battery estimate. It’s easier for the car to estimate SoC accurately after charging rather than during charging.
 
Good point. Didn't check that multiplication while I was walking the dogs today. That modifies all the numbers above a bit....
Good point. Didn't check that multiplication while I was walking the dogs today. I just assumed it was correct. That modifies all the numbers above a bit....

Just double checked my spreadsheet and scratched my head until I realized I pasted a screenshot with an error I later corrected. The actual charge should not be 30.3 or 31.6, but rather 33.0. 31.6 is the actual amount, but I grossed it up because it charged to 237 rather than 247 (so I multiplied by 247/237 to better reflect what it should have consumed to get back to pre-commute state). But, I posted the screenshot where I had the cells reversed and actually brought it down. The corrected version still rounds to $.06 per mile, which might be why I didn't pick up my error when I posted...
 
237 rather than 247 (so I multiplied by 247/237 to better reflect what it should have consumed to get back to pre-commute state

That is incorrect. Multiply by 133/123. That is what it would have been to get back to the original state. It is proportional to the added miles not the total miles.

So 34.2kWh. I have some doubts about your efficiency, but you can check that separately. That could cause the kWh number to be even higher depending on your utility voltage.

In any case we are getting close to the 7 cents/ rated mile estimate not accounting for vampire. (EPA 29kWh/100mi * 23 cents/kWh = 6.7 cents / rated mile)

I guess I should have done this checking this morning at a computer. Would have caught this error. Harder on a phone when walking dogs!
 
Last edited:
  • Like
Reactions: darth_vad3r
That is incorrect. Multiply by 133/123. That is what it would have been to get back to the original state. It is proportional to the added miles not the total miles.

So 34.2kWh. I have some doubts about your efficiency but you can check that separately.

I guess I should have done this checking this morning at a computer. Would have caught this error. Harder on a phone when walking dogs!
Yes, you're exactly right. Good catch. Should be 34.2. I corrected test 1 for this - rounded figure changed to $.06 / mi from $.05.
 
Last edited:
  • Like
Reactions: AlanSubie4Life