The Plug-in Energy Diet

Smart Charging Can Save

Smart Charging Can Save

By Tom Bartley (9/21/09).

It’s not as simple as it first appears to know how much money it’ll take to feed a new plug-in. A modern efficient electric cars with braking regeneration will consume less than 300Wh/mile. The new all electric Nissan Leaf is claiming 100 mile range with a 24kWh battery. Taking into account the battery losses, that’s excellent performance at less than 240Wh/mile. In terms of miles-per-gallon (mpg), as calculated by the GM Volt people, the Nissan Leaf achieves infinite mpg because it doesn’t use one drop of gasoline.

But really, you say, how do I compare out of pocket costs and real energy consumption? There are other perspectives, but have faith and follow along for an analysis to impress your friends.

The heat energy of gasoline is 36.6kWh/gallon, but the efficiency of a gasoline engine reduces this to about 10kWh/gallon in terms of the mechanical energy coming out of the engine. For a reasonable non plug-in high fuel economy car of 30 mpg that’s 30 miles per 10kWh or 366Wh/mile, also pretty good performance. In terms of energy efficiency the power company does a bit better in delivering the kWh energy than does the gasoline engine of my car. The picture is even better when energy security and greenhouse gas reductions are considered.

Note: I’m describing my around town commuter needs here, not any long distance trips. Batteries and energy storage are still a long way from coming close to the energy density of liquid fuel. I’ll discuss range anxiety and battery life in a future post.

Now watch closely; this is the part that matters. How much of my spendable cash is going for fuel to move me and my car down the street?

For a non plug-in at $3.00/gallon and 30mpg, I’m spending 10¢/mile.

For a plug-in at 13¢/kWh base rate charging and 0.24kWh/mile, I’m spending 3.12¢/mile.

The plug-in wins hands down and the margin gets even wider if I happen to live in an area with cheaper electricity. For a frequently used national average number of about 8¢/kWh, I’m only spending 1.92¢/mile.

Before you plug-in fans start clapping and cheering, did you notice the slight of hand? Here is more of the story. What is “base rate charging”?

For most of the 138 million electric energy consumers in the country the electric utility company reads the meter once a month and sends out a bill. Everyone pays the same price, the base rate, for the first kWh after the meter reading.
However, if a lot of electricity is used during the month, such as for air conditioning in the summer or heating in the winter, the price of the last kWh before the meter reading could be 3 or 4 times higher than the first kWh. This is called tiered rate tariffs and is analogous to the graduated income tax rate. The more electricity used the more charged per kWh.

The cost to the utility of generating power varies during the day as extra assets are brought on line to meet the demand. To cover the extra cost the utility companies use the monthly tiered rate pricing structure as approved by the Public Utility Commission (PUC) in each state. The monthly tiered rate pricing doesn’t match up well with the daily cost variations.

Charging my plug-in vehicle would accelerate the higher rate tiered rate tariff and by the end of the month, using the numbers from above, I could be paying 6¢ to 13¢/mile for my plug-in.

The solution is Time-Of-Use (TOU) pricing. Smart meters provide the technology for TOU pricing to match the daily cost variations and the utilities love it!  8.3 million (6% of the meter population) of these meters are already installed nationwide and my utility, SDG&E, is planning for TOU pricing to start in 2011. In fact, a smart meter is integral to the new charging units being installed at home, work, and other locations. If I take delivery of a Nissan Leaf in 2010 I can probably get TOU pricing early.

So now I can have some real choices for how much I have to pay to fuel my electric car just like I have choices for which gas station to use to fill my tank. Ideally, I will do all my charging in the middle of the night for the lowest base rate. The computers in the car and the charging system can take care of that. Again the utilities love it because they don’t have to worry about adding assets to meet my demand.
I’m also thinking about doing a little arbitrage, using the battery in my car to store cheap energy and use it in my house when the grid energy is expensive. More about this in a future post. In the next post I’ll also discuss charging issues, maintenance, and intangibles.

You probably caught that I didn’t talk about the cost of replacing my battery.  I’ll cover that more in a future post, but here’s a hint – the battery will probably last at least 10 years for over 100,000 driving miles.  Present cost estimates are about $300/kWh or $7200 for the Nissan Leaf which adds 7.2¢/mile to the cost of operating my plug-in, but again, it “ain’t” that simple.  Stay tuned.

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About Author: Tom Bartley

Tom Bartley is an expert about vehicles, drive systems, and energy storage. Tom Bartley has a BSEE and MSEE from Stanford University. He is a life member of the IEEE and a member of SAE. He is a director on the board of the San Diego Clean Fuel Coalition and is working with Transpower of the development of heavy-duty electric drive systems.