Car energy use report vs. electric bill

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#1
Just got my second full month electric bill after getting my Model 3 LR RWD. Both months so far show an increase of about 400kWh over last year (with a similar number of cooling degree days) whereas the car reported about 225kWh of usage for the month (using the trip odometer). I charge every night on a 120V/15amp outlet and my commute is about 30 miles round trip. I understand that the car is taking charging efficiency into account, but it still seems like a large delta. Has anyone else tracked this? I’m getting a 14-50 outlet installed soon. Will that help?
 

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Ed Woodrick

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#2
That's a really efficient month, 213 Wh/mi.

Let's do another calculation with your electric bill. 400kWh * about $0.15 = $60
$60 for 1,017 miles isn't worth worrying over, is it?

1017mi / 20 mpg * $3 /gal = $150 for an ICE.

Write your numbers down and remember them, because you are headed to winter and that's going to impact your driving efficiency. You'll be wishing you could get the same numbers.
 
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#3
I’m not too worried about the overall cost. I just wanted see if these numbers were consistent with what others were seeing, or if my charging setup and/or car phantom drain were causing the delta or are worse than normal.
 

GDN

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#6
I don't get into all of the calculations, but there is a long Home Charging thread I read through. Might look from this post Home Charging forward. Charging efficiency has come up in just the last 10 posts or so, if not prior.
 
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#7
I don't get into all of the calculations, but there is a long Home Charging thread I read through. Might look from this post Home Charging forward. Charging efficiency has come up in just the last 10 posts or so, if not prior.
Ah thanks. That had some of the info I need. I had checked that thread but should’ve checked one page back. Looks like I’m likely only getting 75%. Charge efficiency. With that in mind my numbers are closer to making sense. Getting the 14-50 looks like it’ll jump my efficiency to 90-95%.
 
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#8
Charging efficiency is about 85% on my Tesla wall charger.

Ontario, Canada prices:
Electricity ~ 10cent/kWh
Gas~ 130cent/l

In my case, I am averaging 142Wh/km /0.85eff ---> 0.167kWh/km ----> 1.67cent/km or 2.67 cents/mile

Gas car, averaging 30mpg ~ 7.84l/100km ---> 10.2cent/km or 16.3cent/mile

So, it turns out that in Ontario it is about 6 times cheaper (as far as energy consumption) to drive Tesla 3 then a reasonably efficient gas car. I charge my car at night, when electricity price is actually 8.5c/kWh, but I rounded up to 10c/kWh for the energy delivery charges, etc...

I understand that the energy consumption will increase at least 40-50% in the winter, but that will still be at least 1/3 price of a gas car.

Again, these are prices in Ontario, elsewhere might be totally different story. Each case is different, so there is no point arguing what is economically better.
 
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#10
I used 6 AWG wire to the WC, which is around 15 ft run. I have a 50A circuit breaker. I normally charge it overnight at 25 A only because I don't need to charge it faster. Max charging rate of my WC is set to 40A.
 

PNWmisty

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#11
I used 6 AWG wire to the WC, which is around 15 ft run. I have a 50A circuit breaker. I normally charge it overnight at 25 A only because I don't need to charge it faster. Max charging rate of my WC is set to 40A.
You might get a little higher efficiency if you charge at a higher rate.
 
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#12
The reason why I charge at lower rate is to prolong battery life. I think it is better on the battery if charged at lower amperage. Who knows... probably not a big difference between 25 or 40 amps for either charging efficiency of for battery life. If anyone has some concrete info, I’d be good to know what the ideal charging power should be...
 

garsh

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#13
If anyone has some concrete info, I’d be good to know what the ideal charging power should be...
Yep - at these levels, there's no difference. You have to go up to supercharger current levels before it will affect battery life.

There's no appreciable benefit. It'll produce less waste heat, but will do so over a longer period of time, resulting in slightly more wasted total energy. Even at 48 amps, charging current isn't high enough to affect battery lifetime:

https://www.tesla.com/blog/bit-about-batteries
Two outtakes from this article:
  • Avoiding very high charge rates. Charging faster than about C/2 (two hour charge) can reduce the cell's life.
  • Limiting our charge rate is less of a compromise, since the wire size and availability of very high current outlets limit us much more than the batteries do at this point.
This was written in 2006, during development of the Roadster. So even back in the Roadster days, it sounds like Tesla was of the opinion that AC charging even at some of the higher-available currents was still seen as pretty "low" to the Tesla battery pack. They recommended staying under a two-hour charge. The Roadster had a 53kWh battery, and according to wikipedia:
...using a 240 V charger on a 90 A circuit breaker... a complete recharge from empty would require just under 4 hours.
So this seems to support the theory that AC charging at even the highest-possible 48 amp rate isn't going to hurt the battery noticeably compared to charging at a lower current.
 

PNWmisty

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#14
Yep - at these levels, there's no difference. You have to go up to supercharger current levels before it will affect battery life.
True, even at 48 amps it will take 7 hours to fully charge a Long-Range Model 3. A 7-hour charge is barely above a trickle charge and is about as gentle as you can get. So feel free to charge at higher levels without worry.
 

Feathermerchant

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#15
So what does C/2 charging rate mean? For a 5.0 Amp hour battery, 1C means 5 Amps so C/2 = 2.5 Amps. In the model 3 there are 46 cells in parallel for the long range. The batteries have a rumored 5 Amp hours of capacity each. So 5 X 46 = 230 Amp hours. So C/2 for the model 3 long range is 115 Amps.
 

PaulK

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#16
I have a “Juicebox” wall charger which reports total current delivered to the car, and I charge at 40A. Seems I get about 80-85% efficiency. I can’t compare this against my utility bill because my other household usage is too variable.

I installed a 100A breaker then 36” of 2AWG to a sub panel which has a single 50A breaker then 24” of 6AWG copper to the juicebox. The 50A breaker gets quite warm to the touch while charging. I wonder how much efficiency loss I’m adding due to my sub panel. Maybe I should have just used the 50A breaker in my main panel straight to the juicebox?
 

PNWmisty

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#17
I have a “Juicebox” wall charger which reports total current delivered to the car, and I charge at 40A. Seems I get about 80-85% efficiency. I can’t compare this against my utility bill because my other household usage is too variable.

I installed a 100A breaker then 36” of 2AWG to a sub panel which has a single 50A breaker then 24” of 6AWG copper to the juicebox. The 50A breaker gets quite warm to the touch while charging. I wonder how much efficiency loss I’m adding due to my sub panel. Maybe I should have just used the 50A breaker in my main panel straight to the juicebox?
What brand/style of 50 amp breaker do you have? It's normal for them to get a little warm after extended use near their rated capacity but some breakers are simply better than others at transmitting the "juice" without as much resistance (which translates to less warmth and lower power bills). Someone has to pay for that wasted heat. Is the breaker new?

As a side note: Tripping the breaker while charging current is flowing is always a bad idea. This causes an arc inside the breaker which can pit the contacts and leave a deposit of carbon on the contact surfaces which will then increase resistance (and your electric bill) everytime you charge. Breakers should not be used as switches!
 

PaulK

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#19
What brand/style of 50 amp breaker do you have? It's normal for them to get a little warm after extended use near their rated capacity but some breakers are simply better than others at transmitting the "juice" without as much resistance (which translates to less warmth and lower power bills). Someone has to pay for that wasted heat. Is the breaker new?

As a side note: Tripping the breaker while charging current is flowing is always a bad idea. This causes an arc inside the breaker which can pit the contacts and leave a deposit of carbon on the contact surfaces which will then increase resistance (and your electric bill) everytime you charge. Breakers should not be used as switches!
It is a “Square D” brand breaker.

I never switch the breaker on or off, certainly never during charging.

I noticed it gets warm as I touched the breaker just to sample the temperature while charging out of curiosity.

I have read that breakers of all brands get warm with this much current. And yes, this means added resistance which I’m sure leads to extra charging losses. That is why I theorize that if I didn’t install the subpanel then I would save some charging losses (which I cannot easily measure).
 

Feathermerchant

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#20
Some breakers are made to be switches.
As far as warmth, Think of a 40W light bulb. If it were in the breaker, it would be too hot to touch very quickly. Probably 5 Watts or so is plenty to make it warm to the touch. At that rate it takes 200 hours (83 days) to use one kWh. It is a pretty small loss that you will never see on your bill.