Quick Mileage Cost Calculation

[darth_vad3r]

I pay 2.79 cents per kwh to charge overnight (time of use).

This is so ridiculous.

LOL, so true.

Approaching the point where if you are close enough, you should drive to @Knightshade ’s place to charge

 

[Tigerkc]

@OP. Not sure if your electric is under PG&E or not, and what rate plan you are on. Off peak for EVA is $0.13452 (summer) / 0.13778 (winter) per kWh, and $0.15633 per kWh for EV2. EVA was closed to new enrollment as of July 1, 2019, so if you are not in the plan then you can only enroll into EV2 plan.

 

[Knightshade]

Yes, that is 0.7 cents/mi, not 7 cents/mi! For you, worrying about charging inefficiency and vampire drain really is completely unnecessary!

Yeah- in fact they offer free charging at work... but since you have to move your car after 4 hours max (or when you stop charging) and given it'd save me like 25 cents tops in a day, I don't even bother doing it outside the rare case I'm taking a long trip right from work and need to leave with the car at 100%

 

[ahd13]

OP, in terms of sources of inaccuracy ...

1) Does your trip home end with any downhill stretches and/or large deceleration stretches shortly before stopping your car?

There can be several miles of added range from regen hidden from you which could translate to you not having used 46 miles off the dashboard. It seems like it’s possible that up to 1 kWh of regen may be hidden from the dashboard gauge’s distance number at any time. It’s not clear to me if this would stay hidden during a recharge event, I would presume not and it would be added back in as the charge progressed which would manifest as your efficiency for that charge session (as measured by increase dashboard range) being better than it actually was.

2) Also, using a trip meter gives better distance accuracy with 1 decimal place versus the odometer with rounded miles introducing as much as a full mile of error.

To quantify these errors:
1) 39 miles added to the range (204 -> 243) while charging is 9.555 kWh (39 mi x 245 Wh/mi). The 1 kWh max regen ‘error’ is up to 10% error here.

2) “39” off the odometer measured from OdoA (7958 +/- 0.5mi) minus (7919 +/- 0.5mi) = 39 +/- 1.0 mi. So that’s up to 2.56% error.

That all said, I agree with @AlanSubie4Life that something still seems off even with above sources of error:

Seems you charged at 240 V, 48 A so the efficiency should be quite high, easily in the 90’s.

8.4 kWh x 0.90 = 31 miles of added range for AWD with 245 Wh/mi internal charge rate constant. That’s 20% off your number of 39 added miles.

Something seems quite off with the numbers. I would repeat test again.
Also, doing it over a few days will capture vampire drain costs as well.

You really have to weight it for seasonality too since colder winter temps decrease the battery’s efficiency.

Ultimately it's a small sample and prone to error, but these are the numbers I got.

The process was to charge the car to a specific range, drive it to work and back (that's the 39 miles) and measure the electricity consumption (from the grid) necessary to return the range to the pre-commute amount (I made a small adjustment because the range is never returned to the exact same amount). All that done, this was the result.

Could ignoring decimals have improved the outcome? Sure, or worsened it. Was it a warm day? Yes, I live in San Jose. Is there a downhill portion? Yes, very short, but the segment is uphill coming back, naturally. Could the reported grid usage be inaccurate? Also possible.

Not trying to make any broad claims or generalizations, and I'm not a Tesla fanboy. Just sharing the results of some testing. Certainly, it appears my results were at the fat end of the tail, and that I should be expecting mean results closer to 6 or 7 cents over time. And, I appreciate the thought given to your assessment, which gives me more to think about as I try to better understand the vehicle.

 

[MTSN]

My last daily driver cost around $0.23/mile in fuel alone at $3 a gallon, so I'll be saving roughly $2,500/year in gas savings. Adding maintenance, insurance, and depreciation savings on top is more like $4k/year in overall savings, maybe more. I don't keep anything long, but if I did the Tesla would pay for itself in savings alone eventually which is insane.

 

[ahd13]

@OP. Not sure if your electric is under PG&E or not, and what rate plan you are on. Off peak for EVA is $0.13452 (summer) / 0.13778 (winter) per kWh, and $0.15633 per kWh for EV2. EVA was closed to new enrollment as of July 1, 2019, so if you are not in the plan then you can only enroll into EV2 plan.

Thanks I am with PG&E and I think I will look into the plans again. I ran some simulations and the EV plan was not the most economical for me - don't remember why. Two factors to consider are that I have a large solar array and offset my grid pulls, and my wife can charge at work for free (when she remembers!), so it's economically less pressing for me. Still, I should reinvestigate...

 

[darth_vad3r]

Ultimately it's a small sample and prone to error, but these are the numbers I got.

The process was to charge the car to a specific range, drive it to work and back (that's the 39 miles) and measure the electricity consumption (from the grid) necessary to return the range to the pre-commute amount (I made a small adjustment because the range is never returned to the exact same amount). All that done, this was the result.

Are you charging to 80%? 90% might be a better target for testing due to better accuracy in the voltage discharge/charge curve in the steeper sections of the curve (near higher and lower SoC, it’s flatter in the middle).

My charge seems to be within 1-2km (~ 1 mile on the display) when I charge to the same target. 250 start vs 243 end seems pretty large.

Could ignoring decimals have improved the outcome? Sure, or worsened it.

Yes, that’s what errors do! Given that your numbers seem to have a large error though, something might be missing or you got “lucky” and all the errors added up against you.

Is there a downhill portion? Yes, very short, but the segment is uphill coming back, naturally.

Regarding regen I was specifically curious if there was large regen near the end of the test, if it’s downhill to work and uphill back, that would lessen the chance of that specific thing being a possible factor.

Could the reported grid usage be inaccurate? Also possible

Curious. How do you see this usage anyways?

 

[ahd13]

Are you charging to 80%? 90% might be a better target for testing due to better accuracy in the voltage discharge/charge curve in the steeper sections of the curve (near higher and lower SoC, it’s flatter in the middle).

My charge seems to be within 1-2km (~ 1 mile on the display) when I charge to the same target. 250 start vs 243 end seems pretty large.



Yes, that’s what errors do! Given that your numbers seem to have a large error though, something might be missing or you got “lucky” and all the errors added up against you.



Regarding regen I was specifically curious if there was large regen near the end of the test, if it’s downhill to work and uphill back, that would lessen the chance of that specific thing being a possible factor.



Curious. How do you see this usage anyways?

Yes, I have the charger set to around 250 miles, which is close to 80%. I try to avoid charging to 90% as it (according to most experts and E.M. himself) degrades the battery faster. I wasn't aware that the stated-range accuracy would vary by charge percentage. So, that would be another potential source of error. As for the grid usage, my solar app provides information on what is being pulled and fed. In the past, I double checked this data against both my inverter and my utility bill for accuracy and found it to be spot on (I corrected for the 0.4kWH rate that powers my home at night).

 

[powaking]

Well, you need to divide by .83, or whatever charge efficiency your average is, to get wall kWh used.

This is assuming the Wh/mi number you mentioned is one pulled from the car’s trip meter (which is measuring DC kWh out of the battery, after losses from DC kWh into the battery, after losses from AC kWh in from the wall (or DC supercharging)).

So if you are trying to calculate a cost in kWh from AC, you should divide the DC kWh used according to the trip meter by anywhere from ~0.6 to ~0.9 (multiply by 1.11 to 1.67) depending on the AC power you charge at.

So saying the Model 3 should be better than your numbers quoted is missing the multiplier, so the final AC kWh cost might actually be higher than the numbers you quoted (~6-7c/mi) for an AWD 3.

Accounting for inefficiency it’s about ~8-9c/mi. Still much cheaper than the minivan it replaced. Don’t see why the Trip screen within settings can show you the tru fuel savings or running charge cost. There’s plenty of space to display that information.

 

[Bill Price]

My last daily driver cost around $0.23/mile in fuel alone at $3 a gallon, so I'll be saving roughly $2,500/year in gas savings. Adding maintenance, insurance, and depreciation savings on top is more like $4k/year in overall savings, maybe more. I don't keep anything long, but if I did the Tesla would pay for itself in savings alone eventually which is insane.

Quick computations say your former daily driver was getting 13 mpg and you were driving approximately 11K miles a year? Just think how much you would be saving if you were a traveling salesman or manufacturer's rep driving 25K a year or more?

 

[MTSN]

Quick computations say your former daily driver was getting 13 mpg and you were driving approximately 11K miles a year? Just think how much you would be saving if you were a traveling salesman or manufacturer's rep driving 25K a year or more?

Yep exactly. It was a fully built (lift, steel bumpers, steel armor, etc.) Toyota Land Cruiser, and 13mpg was just about all it could do. The Tesla makes a lot of sense with average miles, but the more miles one drives the savings really pile up.