18 Months
Driving on Sunshine

September 24, 2013

Mark D Larsen

Sunny Disposition
(Click to enlarge)

Several weeks ago I shared a version of the above pie chart and tables with fellow EV advocates. Some of them wanted to know what stats and formulas I had used to produce them. To accommodate their request, I will provide below a more detailed explanation using the stats after a year and a half of driving my Leaf. There are basically four sources for the data: my electric utility bills, my solar monitor, my CarWings data, and the EPA fuel economy website. Beyond that, it is a simple question of mathematics.

Rocky Mountain Power monthly bill

My bill reports how much electricity I have drawn from the grid each month, and how much I have put into it with my solar panels. I only have one meter on my house, but it is programmed to tally those amounts separately, as though there were two meters. For example, my last bill reported that Meter 24 (solar) put 601 kWh into the grid, and Meter 14 (utility) pulled 941 kWh from it. As you can deduce, because of the heavy use of air conditioning plus several days of cloudy weather in August and September, I actually used more electricity than my panels generated. But that is an exception to the general rule. During most of the year the opposite occurs, in which case I am able to deposit the extra kWh into a “savings account” to pay for months like this last one. Here are the grand totals since my solar array was installed at the tail end of 2010 until my electric bill on September 20:
Meter 24 into the grid: 27,012 kWh
Meter 14 from the grid: - 21,673 kWh
Excess solar electricity generated: 5,339 kWh

As for the cost of electricity, Rocky Mountain Power charges a standard rate of 8.6¢ per kWh in my state. That price is notably lower than the national average of 12¢ per kWh. There is also an optional tiered rate, called a “time-of-day” plan, but I have never bothered to sign up for it, since I produce more electricity than I use. For EV owners who do not have solar, or not enough to generate all their electricity, here is a comparison between the standard and time-of-day plans.

Sunny Beam solar monitor

This small bluetooth device communicates with my array’s inverter and maintains a record of the kWh generated daily. I copy the data to my computer to create the reports and graphs on my solar pages. The top row in the following table shows the total kWh produced as of that last utility bill. To determine how many of those kWh I have used in the home during the day, I simply subtract the Meter 24 tally above from that total:
Total solar electricity produced: 39,252 kWh
Meter 24 into the grid: - 27,012 kWh
Solar electricity used in the home:: 12,240 kWh

I should add that my 8,280 watt array consists of 36 Trina Solar 230-watt panels. Because they are mounted in clusters at slightly different angles, they never produce their maximum output, but I have occasionally seen the SunnyBoy 7000US inverter reach its peak capacity of about 7.1 kWh in May and June. To date, the most that my system has generated in one day was 59.54 kWh on May 30, 2011; the least was 2.43 kWh on a very stormy January 23, 2012.

CarWings data

As all 2011-2012 Leaf owners know, every time we start the car, the display asks us to authorize CarWings to transmit, record, and store our driving data. The 2013 model only requests such authorization once every thirty days. I wish that the latest software update for my Leaf had included this latter feature, but I have become so conditioned to pushing the OK button that I don’t even think about it anymore. More importantly, I find that the CarWings stats prove useful.

For instance, CarWings shows these distances since I took delivery of my Leaf:

3/28/2012 - 12/31/2012: 6,574 miles
1/1/2013 - 9/20/2013: + 6,077 miles
Total: 12,651 miles

Here are the kWh used to drive the above distance according to CarWings:

3/28/2012 - 12/31/2012: 1,216 kWh
1/1/2013 - 9/20/2013: + 1,076 kWh
Total: 2,292 kWh

From there, it is a simple formula to calculate my driving efficiency to date:

12,651 miles / 2,292 kWh = 5.5 miles-per kWh

Here are the number of days that I have owned my Leaf:

3/28/2012 - 12/31/2012: 279 days
1/1/2013 - 9/20/2013: + 263 days
Total: 542 days

Dividing that number into the total miles produces the average of how far I drive daily:

12,651 miles / 542 days = 23 miles-per-day

A similar formula calculates the average kWh-per-day:

2,292 kWh / 542 days = 4.2 kWh-per-day

Now that we have these data, it is possible to make them even more precise with additional calculations. For example, I always charge my Leaf overnight, so to designate those kWh separately in the pie chart, I deduct them from my utility’s Meter 14 total above:

21,673 kWh - 2,292 kWh = 19,381 kWh

The stats above are what I shared in the original pie chart. However, since fellow advocates want more details, I will render the results even more accurate with an additional calculation. You see, there are always efficiency losses when an EVSE transmits the charge to an EV’s battery pack. From what information I have been able to find, the efficiency is typically 86%. Consequently, here is an estimate of the loss when transmitting the above kWh into my Leaf:

(2,292 kWh / 86%) x 14% = 373 kWh

And since the efficiency loss also occurs while charging overnight, I can likewise designate those kWh in the pie chart by deducting them from the remaining Meter 14 tally:

19,381 kWh - 373 kWh = 19,009 kWh used to power the rest of the home at night

We can now also determine what it would have cost to fuel my Leaf if I had paid for the electricity. Using Rocky Mountain’s rate cited above, this is the result:

(2,292 kWh + 373 kWh) x 8.6¢ = $229

That cost is truly a bargain, thanks to the low utility rates in my area: less than 2¢ per mile! Of course, if we recalculated using the national average of 12¢ per kWh, the result would be nearly $86 higher. Regardless, in my opinion that’s still dirt cheap when compared to the cost of gasoline, as specified below.

EPA data

I consulted the EPA fuel economy webpages to calculate how much much it would cost me to drive my 2005 Subaru Outback as far as I have driven my Leaf. The EPA page for the vehicle reports a combined city/highway rating of 20 MPG. I then consulted this page to determine the price per gallon of gasoline in my area, which averaged $3.60 in 2012 and $3.69 so far this year:
((6,574 miles / 20 MPG) x $3.60) + ((6,076 miles / 20 MPG) x $3.69) = $2,304

That’s ten times more than what the electricity would have cost to fuel my Leaf!

I also used the EPA pages to compare the greenhouse gas emissions between the two vehicles. If you click on the Energy and Economy tab for the Outback, and select “Tailpipe & upstream GHG” via the pop-up menu, you’ll see that the vehicle produces 110 grams upstream and 444 grams from its tailpipe, giving a total of 554 grams per mile. At 0.0022046 lbs. per gram, that translates to 1.22136 lbs. per mile:

(12,651 miles x 1.22136 lbs. per mile) = 15,451 lbs. of GHG

The Leaf has no tailpipe, of course, but the EPA provides a dedicated page to calculate the greenhouse gases produced by generating its electricity upstream, which varies according to utility region. If readers are interested, I recently posted an online tool that calculates the differences among the 26 different utility regions in this country.

When I enter my zip code in the box and click the button, the result is 150 grams per mile. The EPA states on the 2012 Nissan Leaf page that it uses an average of 34 kWh to travel 100 miles. Driving that distance would thus produce 15,000 grams of GHG upstream. Dividing those grams by 34 kWh reveals that the EPA estimates 441.18 grams of GHG per kWh in my region. Since there are 453.5924 grams per lb., the result for my Leaf is:

((2,292 kWh + 373 kWh) * 441.18 grams per kWh) / 453.5924 grams per lb. = 2,592 lbs. of GHG

That is less than 17% of the GHG produced by the Outback!

I’ll be curious to see how these stats change in the future, as prices fluctuate and the grid becomes increasingly cleaner with more renewable energy.

To sum up, I can state unequivocally that driving on sunshine is one of the best decisions I have ever made. True, I figure that, with what I am saving from electric bills and gasoline costs, it will take about 5 more years for the solar array to pay for itself, but after that... it’ll be pure gravy. In truth, however, I really couldn’t care less if I were to end up losing money on these investments, because that’s not why I installed solar panels and bought an EV in the first place. I simply feel a personal responsibility to do my small part in ending our destructive addiction to dirty fossil fuels. There is, after all, no Planet B. Mine is admittedly just one small straw, but with enough straws we can break the petrolcamel’s back.