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Cost per Mile Calculations

 

One of the most common questions people ask me is, "How much does it cost to drive?"  What they mean is, "What is the cost per mile compared to the gasoline version?"  I have read many answers to this question, everything from 2 to 4 cents per mile.  My initial estimate was that my cost was about 2 cents per mile.  I was very wrong!

 

 

What's Needed to Calculate Cost per Mile?

 

To get an 'accurate' figure, you have to 'nail down' as many variables and assumptions as possible - the first of which is the actual cost you are paying for electricity (cents per kilowatthour - kWhr).  Let's make a list of the data that needs to be collected.

  1. You must determine the true cost per kWhr you are paying for electricity (cents/kWhr).

  2. You must take care to observe exactly how many miles you drove.

  3. You must use an AC inline test instrument that can tell you the total energy (# kWhrs) it takes to recharge the batteries.  This device accumulates the energy and displays it for you to see or you can connect it to your computer and graph the charge process and accumulating energy during charge. The 'watts' or 'amps' graph enables you to determine the point at which it is fair to say that charging is finished.  See my graphs below.

With the above information in hand, you can use this formula to calculate the cost per mile.

 

Cost per mile = $0.##/mile <-- value to calculate

Cost per kWhr = $0.##/kWhr <-- taken from your electric bill

Total energy used for charging = # kWhrs <-- provided by AC line test instrument

 

             $0.##/mile = ($0.##/kWhr X # kWhrs) / # miles

 

 

The Real Cost of Electricity

 

As it turns out, my actual cost per kWhr for electrical energy is $0.096 (9.6 cents).  Look carefully at your electric bill to see how many line items are being multiplied by your total energy usage for the month.  My bill shows three line items as follows:

 

 

Bill Line Items

April 2007

April 2008

June 2011

Energy Charges

Energy Management Cost

Bulk Power Cost Adjustment

Total

$0.07524/kWhr

$0.000436/kWhr

$0.0202/kWhr

$0.095876/kWhr

9.6 cents/kWhr

$0.07545

$0.000437

$0.04208

$0.11797

11.8 cents/kWhr

$0.0753

$0.03471
$0.11

11 cents/kWhr

 

Other charges on your electric bill you are going to pay anyway.

Note: The electric rate may vary on a monthly basis if it is tied to fossil fuels as the source of energy.  Such is the case for us, which makes it tricky to calculate cost per mile.

 

 

Actual Case

 

Now, let's apply this to an actual case to see what the cost per mile is.

 

Parameter

April 2007

April 2008

June 2011

Distance driven:

AC energy needed to recharge:

Cost per kWhr of energy:

*Cost per mile

20.9 miles

11.5 kWhrs

$0.096/kWhr

$0.053/mile

20.9 miles

11.5 kWhrs

$0.118/kWhr

$0.065/mile

20.9 miles

11.5 kWhrs

$0.11/kWhr

$0.0605/mile

 

*Cost per mile = [(cost per kWhr) X (number of kWhrs)] / (miles driven)

 

 

Comparing Results to Stock Vehicle

 

How does that compare to the original gasoline version of my truck?

 

Around town, it got about 20 miles per gallon.  At $3.75 per gallon, the cost per mile is $0.1875/mile.

 

That's 18.75 cents per mile.

 

At the $0.118/kWhr electric rate and the $3.75/gallon gas rate, the equivalent miles per gallon that I get with the electric truck is:

 

($3.75/gallon) / ($0.065/mile) = 56.7 miles/gallon  <-- At 56.7 miles/gallon, it's like I'm driving an expensive hybrid - only with far less maintenance over time.

 

 

Comparison Table as Gas Prices Rise (*compare to 6.5 cents per mile, as of April 2008, for the EV version)

 

Gas Price per Gallon

$3.00

$3.25

$3.50

$4.00

$5.00

Stock Truck Cost of Operation per Mile at 20 mpg

15 cents

16.3 cents

17.5 cents

20 cents

25 cents

Equivalent Mileage of Electric Truck

46.2 mpg

50 mpg

53.9 mpg

61.5 mpg

76.9 mpg

*Note: The electricity cost per mile increases because of rate hikes and because of battery aging - it takes longer to charge the batteries as they age.

 

 

Cost Matrix

 

The following Cost Matrix shows the equivalent miles per gallon and assumes 20 mpg for the original ICE vehicle and 0.55 kWhrs/mile to recharge.  However, keep in mind that it costs more to recharge as the batteries age.

 

Electricity
Cost

$2.50/gal.
$0.125/mile

$3.00/gal.
$0.15/mile

$3.50/gal.
$0.175/mile

$4.00/gal.
$0.20/mile

$4.50/gal.
$0.225/mile

$5.00/gal.
$0.25/mile

$0.04/kWhr
$0.022/mile

114 mpg

136 mpg

159 mpg

182 mpg

205 mpg

227 mpg

$0.06/kWhr
$0.033/mile

76 mpg

91 mpg

106 mpg

121 mpg

136 mpg

152 mpg

$0.08/kWhr
$0.044/mile

57 mpg

68 mpg

80 mpg

91 mpg

102 mpg

114 mpg

$0.10/kWhr
$0.055/mile

45 mpg

55 mpg

64 mpg

73 mpg

82 mpg

91 mpg

$0.12/kWhr
$0.066/mile

38 mpg

45 mpg

53 mpg

61 mpg

68 mpg

76 mpg

$0.14/kWhr
$0.077/mile

32 mpg

39 mpg

45 mpg

52 mpg

58 mpg

65 mpg

 

 

Still Worth It!

 

Do I still think it is worth it?  Yes!  Why?

  • Great mileage

  • Low operating cost

  • Low maintenance cost

  • Simple system

  • Energy cost is included on my electric bill.

  • No gas station stops and avoids possibly long gas lines and rationing.

  • Reduces oil/gasoline demand which helps keep gas prices lower.

 

Graphs (early 2007)

 

 

Total Watthours

The above graph shows the watthours accumulating over time.  The charging cycle ends at the top of the blue graph where it curves.  For the calculations we did above, I rounded the watthours up to 11,500 Whrs, which is the same as 11.5 kWhrs.

 

 

Charge Current Graph

The above graph shows the current that is drawn from the AC receptacle as the batteries charge.

 

- Mark

 

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