Buzzy Math

By Bill Moore

Posted: 10 Nov 2010

What are they smoking over at Edmunds? First they claim the Volt only got 31 mpg in their test drive. Then they claim, as a result, that using electricity to power the car is more expensive than burning gasoline in it.


Oh sure, they support their claims with all sorts of specs and numbers, including the caveat they paid 31¢ a kilowatt hour for their electricity. But asserting that burning gasoline is cheaper than electricity is, well... buzzy math, in my book.

Okay, I live in a state with some of the lowest electricity rates in the nation; and I don't question Edmund's integrity, but how in the world they are paying 31¢/kWh, I haven't a clue. The highest average rate in California is half that: 15.3¢. Now maybe they charged the car at the highest time of use rate, I don't know; but very quickly, smart consumers are going to figure out that they will want to charge at off-peak hours, not during prime time.

Now, next they claim that the car used 39kWh to drive 100 miles.

Here's my question, were those all-electric miles, broken into three separate drives, or was it over a single driving session. Let's assume, its the latter. The Volt is designed to drive between 25-50 miles as an electric car, and given that Edmunds was probably driving in Southern California, I am going to assume they could have gotten 35 miles in EV range, consuming 8kWh of energy before the car turned on the gasoline engine. For the next 65 miles, the car will be operating in hybrid mode, getting, let's say, 35 mpg just to be conservative. That means the car burned, in addition to the 8 kWh of electricity, 1.85 gallons of premium gasoline to cover the 100 miles. Premium in California is, at the moment, running $3.79 a gallon.

Okay, so 1.85 gallons equals $7.01, plus 8 kWh times 31¢, results in a total trip cost of $9.49. Now let's take the same trip by Chevrolet Cruise, which GM says gets combined city/highway mileage of 38 mpg. It would burn 2.63 gallons of fuel, using lower-cost unleaded averaging $3.11 a gallon. The same trip by Cruze is $8.18. So, in this scenario, the Cruze was cheaper to operate using Edmunds numbers.

But, let's assume instead that the 100 miles was, in fact, made up of three separate trips, each 33 miles distance, which also happens to be slightly more than the average daily commute distance in the LA and Orange Country region. What happens then? Again let's assume the Edmunds' drivers were heavy footed and ran the AC unit and got 33 miles of electric range before the car shifted to gasoline hybrid mode. Again the Volt's software automatically starts the engine when the 16 kWh battery pack uses half of its energy, or 8kWh. So, each trip would consume 8kWh times three or 24kWh. At their exorbitant 31¢ rate, the 100 miles should have cost $7.44 in electricity. How they managed to inflate this to 39kWh, I haven't a clue. I can't believe the Volt's battery system is that inefficient.

But let's assume for the moment that through totally careless driving, they, in fact, did consume that much electricity, what would happened if they simply bought power at the average power rate in California of 16¢, not even at cheaper-yet off-peak rates between 11 PM and 7 AM? Those three trips combined would now have cost Edmunds just $6.24. Also, it's around this point that installing solar panels starts to make great economic sense and cents. Once the PV system is paid for, in say, seven years, your 'fuel' is free for the next two decades, at least. You can't say that about gasoline now, can you?

What I take away from Edmunds little exercise is this: One, they did not drive the car with any thought to efficiency; and Two, they paid an amazingly exorbitant rate for the electric power they did buy. In effect, theirs is, in my personal view, a 'worse case' scenario for the Volt. If this is how California drivers expect to utilize the car, then they might as well buy a Cruze, or better yet, a Prius. However, if they plan to operate the car with any semblance of care and responsibility, it will reward them back with years of gasoline-free driving.

It's your choice. You might also want to look into Time of Use (TOU) metering?

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