Additional Math Pages & Resources

Monday, November 9, 2009

Is electricity free in the car?

Recently my wife asked if the daytime running lights on our Volvo reduced the fuel economy. It seems likely that they do cost something to run, but how could we tell what our cost really was?

One source estimates the cost at a penny per day. That doesn't seem like too much.

Last week I had the chance to talk to some folks from Sylvania. They assured me that  headlights on a car are expensive to run. Last month they introduced EcoBright energy-saving bulbs. They predict these bulbs will save up to $20 in fuel over their usable life. In our case, that's only two years, because they are on all the time.

This raises the question - How much does electricity really cost in the car?

Doesn't it come for free when the engine's running anyway?

No, not really. This took a lot of digging to resolve, but here's what I learned:

Electrical energy in your car comes from the alternator, which driven by a belt off the engine. A diesel engine is about 40-50% efficient in converting gasoline into horsepower; gasoline engines are about 20-25% efficient. More efficiency is lost in the belt drive, and about 50% more is lost converting the rotation to electricity in the alternator, and a bit is lost sending it to the lamps or the battery.

Lots of websites have uninformed users pontificating about the inefficiency of this process, saying I guess this is how it works or my opinion is ... and that's not good enough for me. I finally found a real expert, Mike Bradfield at Remy (formerly Delco-Remy and Delphi). His biography says that over 25 years working in automotive electrical systems, he has been responsible for the initial product engineering design that has led to over 3 billion dollars in sales and the receipt of 24 patents.

Now that's an expert in my book!

Mike has recently written a 32-page paper on efficiency of vehicle charging systems. It's focused mainly on school buses and delivery trucks that are on the road all day. I read it, to save you the time and trouble.

The bottom line is this:

At 4 dollars per gallon, electrical energy costs 52 cents per kilowatt hour. 

Or to state it another way, for each $1 worth of fuel cost, making electricity in your vehicle costs you about 13 cents (the same as the baseline rate here in San Diego).

What does that mean? With the current price of fuel around $3.00 per gallon here in Southern California, electricity generated by the car costs 3 times as much as the lowest rate offered by the utility company.

A truck on the road 8 hours a day all year consumes $1000 in fuel just generating electricity. With Remy's new high-efficiency alternators, a trucking fleet can save 10-20% of their annual fuel used to generate electricity. If we do the math, that's about $100-200 per year, against an alternator cost of $500 - pretty quick payback.

Of course as they say, Your Mileage May Vary.
Here's a chart showing the average price of electricity in 2003.
If you want the newer rates unfortunately you have to go look them up yourself.

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