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How "Green" is a Tesla Roadster?

 Tesla Motors is a car manufacturer that is creating purely electric cars. Their first car that is available is the Roadster, a sporty two-seater convertible shown at the left. It is a pure "plug-in" (no gas engine) and costs as of this writing (in summer of 2009) about \$100,000. A somewhat comparable gasoline vehicle is the BMW-Z4 shown at the right. It currently costs about \$50,000.

If you drove one or the other of these cars an average of 1000 miles per month, estimate how long it would take before you would have spent an extra \$50,000 in gas driving the BMW, above and beyond the cost of the electricity to drive the Roadster the same distance.

According to the two manufacturers' websites, the Roadster uses about 135 Watt-hours to go 1 kilometer while the Z4 needs 9 liters of gasoline to travel 100 km. (This is the way "miles per gallon" is cited in Europe -- liters per 100 km. Note this is different from the US. Is a bigger number of a smaller number better in the two cases?)

A reasonable current estimate for prices for these energy sources is \$0.1 per kWh (kilo-Watt-hour) of electricity and \$3 per gallon of gasoline.

Once you have decided how long it would take for the energy savings of the Roadster to "pay off", discuss other factors that might be relevant. (Don't quantify them here, just identify a few other factors you might take into account in decided how "green" the Roadster is.)