Batteries for electric cars are still very expensive and it is common for governments to subsidize their cost to encourage production ramps that should lead to lower costs as well as more R&D that should also lead to lower costs.
One class of vehicle - the plug-in hybrid - has a battery that is large enough for short trips, but a conventional engine is used for longer trips. One can work out a calculus of what the most economical size is given someone's driving pattern as well as which has the most positive environment impact given driving patterns and the source of electric used to charge the car (coal being the worst case and hydro power or other renewables being the best case).
These calculations can get tricky - here is the latest from CMU (highly readable pdf). For current technologies they conclude that a fairly small and inexpensive battery pack has the best economic and environmental impact. Perhaps stimulus plans need to reflect that (in the US tax incentives increase as the battery size increases).
Of course ecars get better environmentally as the source of power on the grid cleans up. This and the falling price of batteries means this question should be revisited on a regular basis.
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