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"The Volt will be the opposite of a Prius in that it is an electric car with a backup gasoline generator, rather than a gasoline motor with electric backup."
Actually, the statement about Priuses is true of Honda Hybrids, but not a Prius. A stock Prius is actually an electric car first (it will run on pure electric power, but will never run on pure gasoline power). It's just that the batteries and electric motor are both so small that they don't support driving above about 25 MPH (40 KPH) and then only for short distances.
The point you're trying to make in general is correct however: A Prius can't be used as a pure electric car in real world circumstances.