Charging Math, Miles Per Dollar - Is this right?

timbop

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Re the power draw from the house (source), does the car know this? I guess not. The EVSE should know it, or seems like it could at least know it. I guess this kind of data is the value-add of some of the higher end EVSEs.
Nope. The car only is able to measure how much went into the battery. If you have a smart charger then it should have a way to measure what is drawn from the grid, but if you don't have one of those you could track your electric meter as an approximation. It won't matter though: in my example above since the Mach-E is so much cheaper to run the 10% overhead is basically just a rounding error.
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It really depends on how your electric bill works. Some people have delivery charges broken out separately, some don't. Mine doesn't. I just have an electricity charge plus a small tax. Then I have a fixed grid connect charge and a streetlight charge that don't change whether charging the car or not, so I don't count those.

I do have a modest demand charge though for the peak 15 minute period of the month. That's trickier to allocate toward car charging because only part of it is attributable to the car. That's part of why set my EVSE at lower power (24A), so it adds less to that demand charge peak.

Anyway, as others have pointed out, count only the incremental charges indexed on kWh usage, and exclude any fixed charges that you pay simply for having electrical service at your house.
 

RickMachE

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Re the power draw from the house (source), does the car know this? I guess not. The EVSE should know it, or seems like it could at least know it. I guess this kind of data is the value-add of some of the higher end EVSEs.
No, the EVSE doesn't know, because the loss takes place between it and the car's battery. Much of that loss is in the cord between the two vehicles.

What an EVSE can tell you is how much it delivered, and you can compare that to how much the Mach-E received. Of course "rounding" can change the numbers a bit.

We were at 59% when I parked it on Thursday, and it sat until 7PM awaiting off-peak hours. It received, per my JuiceBox app, 30.02kWh (actually 30.0128) in 3 hours and 55 minutes. The Mach-E, per FordPass, shows it received 26kWh. That's a loss of 13.39%. Of course the FordPass number isn't to two decimal places, so let's assume it received 26.44kWh. That's a loss of 11.93%. If it only received 25.56kWh, then it's a loss of 14.86%.

So when you compute miles per kWh, which do you use? What you pay for, or what you actually put in your vehicle? If you are tracking efficiency of the vehicle, then you use the smaller number. But, if you're tracking cost, you should use the cost of the bigger number, and not say "I pay 10 cents per kWh and I only used 26." No, you paid for 30.

I drove 72 miles to use those kWh. Using the 26kWh number, I got 2.77m/kWh.

I use a standard of 12% loss at home, vs. looking it up all the time.
 
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Ok, got my geek on and made a spreadsheet. Seem about right?

CumCum
ChargeCharged UseableKwhkWhKWhCostCostKWhCost
AddedToBatteryAddedLoss RateUsed$/KWh$$Used$$
17%74%8850.1612%56.18$0.116$6.5256.18$6.52
54%100%8840.4812%45.34$0.116$5.26101.52$11.78
22%90%8859.8412%67.02$0.116$7.77168.54$19.55
61%90%8825.5212%28.58$0.116$3.32197.12$22.87
65%90%8822.0012%24.64$0.116$2.86221.76$25.72
16%83%8858.9612%66.04$0.116$7.66287.80$33.38
34%90%8849.2812%55.19$0.116$6.40342.99$39.79
58%90%8828.1612%31.54$0.116$3.66374.53$43.45
23%90%8858.9612%66.04$0.116$7.66440.56$51.11
39%90%8844.8812%50.27$0.116$5.83490.83$56.94
Mach-EOdometer953
$$ / mile$0.060
Miles / $$$16.74
F150ave mpg17
gasoline$2.75
$$ / mile$0.162
Miles / $$$6.18

So, not even 3x better than my truck.
Model above assumed received Mach-e charged to 90% at 0 miles.
 

RickMachE

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No, you've got the math wrong.

If I pull 100kW from the house, and only 88 make it to the car, then the math 88/100 -1 = -12%

To calculate backwards from the 88 that made it to the car, the math is 100/88 - 1 = 13.64% gain.

88 x 1.1364 = 100.

So on your line one, 50.16kWh in the car equals 57 at the house, not 56.18.

The other thing that is wrong is quite simple. Your truck tells you how many miles per gallon it gets, and you used that with the cost of gas and computed your cost per mile. That's fine.

However, you need to use that same variable in your calculations above. If the car tells you that you're getting 2.7 miles per kWh, you had to buy 1.1364kWh in reality, at $0.116 each.

1.1364 * 0.116 = 13.18 cents divided by 2.7 = 4.88 cents per mile.

Note that I just made up the 2.7. My point is you can't take the cost of filling the tank with electricity, and divide that by miles driven, because you have an extra tank there, i.e. you filled it to 90% and counted that in the cost, but didn't use it. You have to take the lifetime miles per kWh to do the comparative math to the mpg.

You should end up closer to almost 4 times the cost to run the truck, dependent on the cost of the gas.
 
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Aha, so in stead on *(1+.12) i did /(1-.12) - gotcha.

CumCum
ChargeChargedUseableKwhkWhKWhCostCostKWhCost
AddedToBatteryAddedLoss RateUsed$/KWh$$Used$$
17%74%8850.1612%57.00$0.116$6.6157.00$6.61
54%100%8840.4812%46.00$0.116$5.34103.00$11.95
22%90%8859.8412%68.00$0.116$7.89171.00$19.84
61%90%8825.5212%29.00$0.116$3.36200.00$23.20
65%90%8822.0012%25.00$0.116$2.90225.00$26.10
16%83%8858.9612%67.00$0.116$7.77292.00$33.87
34%90%8849.2812%56.00$0.116$6.50348.00$40.37
58%90%8828.1612%32.00$0.116$3.71380.00$44.08
23%90%8858.9612%67.00$0.116$7.77447.00$51.85
39%90%8844.8812%51.00$0.116$5.92498.00$57.77
Mach-EOdometer953
$$ / mile$0.061
Miles / $$$16.50
F150ave mpg17
gasoline$2.75
$$ / mile$0.162
Miles / $$$6.18

Slightly worse
 

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Where do I find the final kWh "delivered" on Fordpass or the MME screen under charging? That'll be useful to see what I lost between EVSE and MME's battery!

I'm already paying fixed customer utility fees for my main house electric meter, so those charges weren't factored into my EV charges... Only the $5/mo 2nd meter fee. I'm keeping track of my actual mileage too and the range-gained estimate is getting much more accurate now! I'll go into the Grizzle Smart dashboard online and check later on the stat's. I am averaging about 3-3.5 kWh/mile...

My 39 MPG Camry Hybrid costs about $0.11/mi for fuel factoring $4.30/g gas here in So. Cal! And that's at Costco stations! Most anywhere else and it's $4.69+!!
 
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However, you need to use that same variable in your calculations above. If the car tells you that you're getting 2.7 miles per kWh, you had to buy 1.1364kWh in reality, at $0.116 each.

1.1364 * 0.116 = 13.18 cents divided by 2.7 = 4.88 cents per mile.

Note that I just made up the 2.7. My point is you can't take the cost of filling the tank with electricity, and divide that by miles driven, because you have an extra tank there, i.e. you filled it to 90% and counted that in the cost, but didn't use it. You have to take the lifetime miles per kWh to do the comparative math to the mpg.

You should end up closer to almost 4 times the cost to run the truck, dependent on the cost of the gas.
I get your point, but I would also have to include the cost of the very first charge. I did not include the initial charge from the dealer. So, I had to include my last charge to get it to 90%. Assuming the dealer charged to 90%, then it should be about right, a wash, no?

And if it is, then these number show miles/kWh just under 2. Yikes.

Edit - taking off the 12% loss (which you should for this calc) i get 2.17 miles / kwh. Not great.
 
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Where do I find the final kWh "delivered" on Fordpass or the MME screen under charging? That'll be useful to see what I lost between EVSE and MME's battery!
I had to calculate it based on %added, charged to, and useable battery size

My 39 MPG Camry Hybrid costs about $0.11/mi for fuel factoring $4.30/g gas here in So. Cal! And that's at Costco stations! Most anywhere else and it's $4.69+!!
Wow!! Oh my gosh. 'dems crazy prices...
 

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You can get the kWh added after the charge session by clicking on Details on the blue bar in the app, but once it's gone you can't find it anywhere.
 

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Ok, got my geek on and made a spreadsheet. Seem about right?

So, not even 3x better than my truck.
Model above assumed received Mach-e charged to 90% at 0 miles.
OR: Just buy a smart EVSE (like my Juice Box 40) and all the kwh leaving your wall plug are automatically totaled and logged by charging session and cumulatively recorded until you delete them. Then set one of your trip meters at the time of your monthly electric bill, record the average kwh for the month, and record your mileage from your FP app at the end of each billing month. Now you have everything (except your away from home kwh) to calculate your MME cost per mile. FP app should really be improved to include kwh added in addition to % charge added. I have not done the above exercise but my cost per mile on my Premium EX AWD is between $0.04 and $0.05 per mile.
 

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I just charged-up from 25% to 90% this week. My EVSE sub-electrical meter (City property) says I used exactly 65 kWh for this 65% charge! I pay only $0.1056/kWh rate for EV charging & $5/mo for sub-meter ($0.63/charge estimating 8 charges/month). This charge cost me exactly $7.49! Fordpass estimated I gained about 185 miles... So, grand estimated total cost per mile for me is 4 cents! Holly cow!
Hi, I noticed that you're in SoCal. How are you getting $0.1056/kWh? I'm in SoCal and I pay a minimum of $0.19/kWh.

Question for everyone: Is my math correct?
Mach E efficiency = 100 miles/34kWh
Per mile kWh = 2.94
Commute of 62 miles round trip, total kWh for daily commute = 62/2.94 = 21.088 kWh
Total cost of daily commute = 21.088 * 19 cents = $4.00
With 251 working days per year, total annual commute cost = $4.00 *251 = $1,005.70

Is this correct?
 

timbop

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Hi, I noticed that you're in SoCal. How are you getting $0.1056/kWh? I'm in SoCal and I pay a minimum of $0.19/kWh.

Question for everyone: Is my math correct?
Mach E efficiency = 100 miles/34kWh
Per mile kWh = 2.94
Commute of 62 miles round trip, total kWh for daily commute = 62/2.94 = 21.088 kWh
Total cost of daily commute = 21.088 * 19 cents = $4.00
With 251 working days per year, total annual commute cost = $4.00 *251 = $1,005.70

Is this correct?
Yes, for your commute that is roughly correct but as mentioned above add about 10% in overhead for charging. The reason is that the inverters are imperfect, and the components heat up during charging which causes more resistance and energy loss to push the current through the wiring.

So, figure around $1100 to $1150 for the total commute costs.
 

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With delivery costs, ours is 9.5 cents/kWh in the winter between midnight and 5am. It's 7 cents/kWh in the summer overnight. So, that's around 3 cents a mile. My A4 was about 10 cents a mile. The Mach-e is just a fantastic per mile value!
 

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We're talking about two different things. @RedStormREHS was saying how many kWh he sent from the wall to the car. I was pointing out that his car received 12% less kWh than was sent to it (not unusual to have a 7 - 15% loss. Same is true when you DC fast charge. You paid for 50kW, and your car gets 46.5.

As to the cost of that electricity at home, yes, people need to look at every variable cost they get charged on the bill. Many have just one fixed cost - a monthly customer charge or meter charge. Then there can be 1/2 dozen variable costs, including generation, delivery, paying for investments like a nuclear plant, etc. Some have a higher unit cost when they exceed a certain amount of usage, and that marginal cost should be used. And then there is sales tax.

Here's my bill for a given month. All my vehicle charging is done off-peak.

So my low $0.0116 per kWh cost has to have added to it $0.0426 plus $0.00322 plus $0.066110, plus a portion of LIEAF Factor and Other Delivery Charges, plus sales tax. That takes my 1.16 cents to over 12 cents.

dte.PNG
To get your low cost off-peak rate did you have to agree to a higher cost for other time of day rate usage.

To get a low off-peak rate my power company wanted to increase the normal rate as part of the offer. As we are home during the day we use enough power for lights, electronics, and cooling that the off peak rate savings would not have made up for the increased non-off-peak rate
 
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