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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.
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:
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.
Now, let's apply this to an actual case to see what the cost per mile is.
*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)
*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.
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.
Still Worth It!
Do I still think it is worth it? Yes! Why?
Graphs (early 2007)
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.
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