are electric cars good or bad for the environment?

Disclaimer: I am not an engineer of any sort.
I have been watching the debate over electric vehicles, specifically whether
or not they are actually benefical to the envirnoment, presuming they are charged using electricity from coal plants. My question is more about the plants themselves, and how much electricity that is generated by them is actually consumed.
When I turn off a light bulb in my house, does the coal-fired generator that provides my electricity suddenly see that it needs to burn less coal? I doubt it. I am beginning to surmise that a plant must generate more electricity than is actually used. But, where does this excess juice go? And since power rates are lower at night, does this mean that even more of the generated power goes unused then?
All of this leads me to a belief that electric cars, especially those charged at night, are not responsible for the burning of any additional coal, due to the (presumed by me) over-production that it must maintain. They just help to make use of a higher percentage of the electricity that is being generated anyway.
Am I off-base here? Someone please put an authoritative spin on all of this!
Thanks much, Steve
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On Tue, 18 May 2010 14:30:23 -0400, "Steve"

The generators generate voltage. Whatever current flows is a result of what the load is, so if the current demand of the load goes down, then so does the generated power.
It's most efficient to size things to run at 100% all the time rather than 150% now and 50% later, as there's always fixed losses.
Another interesting thing to think about is I-Squared-R losses in the distribution wires.
If there are two equal loads night and day, then the energy losses are I^2*R*T + I^2*R*T = 2*(I^2*R*T).
But if both loads at the same time, the losses are (2*I)^2*R*T 4*(I^2*R*T). So you've doubled the line losses.
If example, lets say each load is 10 amps for 8 hours, and the line resistance R is 1 ohm. By my calculations, If the loads are split, the total energy loss is 5.76 MJ. If the loads are applied together, the total energy loss is 11.52 MJ.
Another thing to think about--electric cars allow the use of alternate energy sources such as wind, geothermal, solar, nuclear to displace some of the oil usage. .
dave y.
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Steve wrote:

Turning off a single light bulb is far too small a load to have a measurable effect on generator output. The amount of fuel consumed to drive the generator does vary with load however, typically being much greater during so called peak hours during the day and falling off at night. As someone else said, generator efficiency does drop as the output falls below 100%, so there is some effort to shift loads to off-peak hours where there is excess capacity.
As for electric cars, the debate has raged on since the beginning, and there are far too many factors to say one way or the other with certainty. On one hand a large scale power generation plant is far more efficient at converting fuel to energy than individual combustion engines in cars. On another hand, electric cars tend to require considerably more energy to manufacture, there is loss in every step of transmitting and transforming the power, and you have the issue of recycling the batteries when they wear out.
My opinion is that electric cars are not an end-all solution to our transportation problems, but they are ideal in certain situations where they fill a niche, and they drive the development of technology that will be useful in one form or another in the future. Over here where a large portion of our power comes from hydro, they make reasonable sense. 95% of my driving is the 8 mile commute to work and picking up groceries and such around town. I wouldn't get rid of my gasoline car, but even something with a 20 mile range between overnight (off-peak) charges would cover the vast majority of the driving I do. Removing local sources of air pollution in congested city streets is a good thing.
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Well Steve, concerning excess electricity, it is sometimes sold to places that could do with a bit more. (For example here in the UK we sell electricity to France and vice versa) Some immersion heaters are also capable of being remotely controlled, so if there is an excess of electricity the power companies will turn on your central heating to use it all up. You still pay for this extra power your home uses although it is given to you at a discount.
Sorry this was a little off topic but the more you know!
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