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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.
-
You
must determine the true cost per kWhr you are paying for
electricity (cents/kWhr).
-
You
must take care to observe exactly how many miles you drove.
-
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
|
|
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 |
Other
charges on your electric bill you are going to pay anyway.
Actual
Case
Now,
let's apply this to an actual case to see what the cost per mile
is.
|
Parameter
|
April
2007
|
April
2008
|
|
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 |
*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.
Still
Worth It!
Do
I still think it is worth it? Yes! Why?
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