Your mileage WILL vary

by Ernie Hernandez on April 2, 2011

LEAF Headlight

Everyone’s situation is different

We had a reader ask us why we compared the LEAF with the Quest in yesterday’s article. The answer is simple – in our household the LEAF is replacing the Quest as our primary vehicle. The Quest replaces the Sentra as our secondary vehicle. This will not be the case for many, perhaps most, but it is the case for us. But there is more that needs to be discussed regarding yesterday’s article.

In order to make projections into the future, assumptions need to be made. We provided our assumptions in the article. These included the following assumptions:

  • Unchanging energy costs based on current energy costs
  • Future driving habits consistent with existing driving habits
  • Future vehicle use based on past vehicle use
  • Energy loss of over 11% from the wall to the battery (use of 27 kilowatt hours to charge a 24 kilowatt hour battery pack)
  • 3.125 miles per kilowatt hour (or 75 miles per full charge)

None of these are written in concrete, and all of them are likely to be inaccurate to some degree. So why did we use them? Because when you make projections, you need to make projections based on something. The cost of energy is likely to increase – both electricity and gasoline. Which one will rise more over the next nine years? Nobody knows. Don’t believe them if they try to tell you otherwise. Here is a graph of U.S. Residential Electricity Price from the Department of Energy for the past ten years:

US Residential Electricity Price 2000-2010And here is a graph of the average of all grades of gasoline in the United States for the same time frame:

Retail Gasoline Prices 2000-2010So over the past ten years, the cost of gasoline has more than doubled. The cost of electricity has also risen, but by a lesser percentage amount. Will this differential remain to be the case? It’s anyone’s guess. All we can do is look at what has happened in the past.

We tried to be equally conservative with our other estimates. While we are currently charging using the supplied level 1 120-volt EVSE, we don’t know what our own charge efficiency is. In looking at ECOtality’s information, it seems we may have been optimistic regarding EVSE efficiency. ECOtality’s own projection is EVSE efficiency of 85% (found at the bottom of the presentation). That said, it appears that we were conservative in our estimate of 3.125 miles per kilowatt. Based on our own LEAF data to date, we are achieving “Average Energy Economy” of 3.4 miles per kilowatt hour, or almost 10% better than projected. This likely offsets the optimistic EVSE efficiency and would equate to roughly 82 miles from a full charge vs. the 75 miles per full charge that we mentioned in the article.

Again, we want to stress that this is in no way meant as a guideline for anyone else to use. It is just a presentation of information based on one family’s history of previous vehicle usage and potential future vehicle use.

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