It is space and weight - it' called energy density and it is of key importance when designing motive power. If your fuel energy density is not high enough then your vehicle will never be efficient or viable. See http://wiki.xtronics.com/index.php/Energy_density for an energy density comparison between different fuels - note the disclaimer at the top...
be nice if battery prices were coming down - other than at a linear crawl; about the same as usable battery capacity is going up - slowly.
From the Tesla web site the sports version will draw 40A at 220 Volts if your house (most UK houses have this) has a 50A main breaker. That leaves you 10 amps to run heating/washing machines/dishwashers/lights/TVs/Computers. It also takes 6 hours to charge.
This gives us a demand of 8800 watts (220V times 40Amps).
This gives us a figure of some 52.8KW/hours capacity (8800 watts times 6 hours) when new.
No figures appear available for how the battery ages - but if we are kind we can assume it is no worse than a loss of 20% capacity after 5 years - ie 20% less range
The domestic grid in the UK is planned around a concept called 'Diversity'. i.e. not everyone will draw 50 Amps at once. The Coronation Street adverse effect is I presume known - the National Grid puts another power station on line when the adverts come on and people make a cuppa ?
I wonder what is going to happen when a few hundred Electric cars start drawing 40 amps each from the same sub-station ?
If we already have so many people in fuel poverty how are they supposed to handle this; they can't afford to put a 1KW electric heater on; never mind 8.8 KW so they can use personal transport.
Problems to solve before an electric fleet is viable :
Grid Infrastructure - how to supply DOMESTIC premise with enough current capacity to charge cars
Grid Infrastructure - how to generate all the extra KW needed ?
Battery - better energy/weight (energy density)- currently coming down slowly has achieved an energy density about 10 times worse than petrol.
Battery - cost
Battery - manufacture of toxic device
Battery - disposal of toxic substances
Note just under 19 cars require a MW of power - one 1MW capacity wind turbine in the UK provides on average 24% of capacity. - so 19 cars need provision of FOUR medium sized Wind Turbines (and a conventional power station to have 1MW excess capacity for when the wind don't blow/blows too hard)
At the moment the whole schema does not stack up; and we need some pretty major advances before it does.