But when it comes to cars, a century of petrol and diesel vehicles has left us with the mindset that a car has to be refuelled with energy which is then used up as it travels along. For a battery electric car, this means comparatively low cruising speeds and/or a short range compared to conventional cars, as we currently have no means of storing electric energy at anything like the low cost and high density we can store chemical energy in a petrol tank.
So here is my favourite solution to the problem of how to replace conventional cars with something that will run on renewably-generated power.
This isn't a new idea, but an old one that deserves another look. Trolleybuses are an old technology, still in use in places such as Seattle. These are simply electric buses that take power from an overhead cable, using a pick-up mechanism on a long flexible pole. They can drive all day like this, quietly producing no pollution, and readily using whatever source of electricity supplies the grid.
In particular, renewably-generated electricity can be used with excellent efficiency. This is because no energy is wasted turning the electricity into a fuel, storing it, and converting it back into electricity to power the motor. This scores points over the solution of using renewable electricity to generate hydrogen to use in fuel cells, though that route certainly has its merits.
Many communities are now utterly car-dependent, and something like the car is going to be with us for a long time now. Making them work more like trolleybuses presents a few problems:
But once the car leaves the motorway and is pootling about town, it is fine to depend on an internal battery with a range of a few tens of miles, and a more modest top speed. This way, no special infrastructure is needed on small roads. A smallish battery keeps the weight and cost of on-board energy storage reasonable, and avoids the vicious circle of better range means bigger battery which means more weight which means more power which means a still bigger battery...
And of course, we might still supplement the infrastructure with some static charging points, e.g. in residential streets and domestic garages. This has the bonus of providing a potential buffer for intermittent sources of renewable electricity.
Note that switching the car fleet to hydrogen or methanol also has a huge infrastructure cost. At least we already have an electricity distribution network.
I would instead suggest something quite low-tech, such as alternating metal strips on the road surface carrying voltages high enough for reasonable power transfer, but too low to be dangerous (e.g. about 60V). Cars could automatically lower metal pick-up wheels to make contact with the conducting strips. While contact might be patchy and prone to interruption by debris or poor tracking, this does not matter much if the car has a battery to buffer the supply.
Similarly, one could envisage parallel metal walls (at least for a highway lane) which vehicles could extend pick-up wheels outwards to touch. Or an overhead wire with a train-style pantograph lifting up to touch it, though of course tall and short vehicles could not then share the same lane.
Sections of electrified road would require protection circuitry to shut them down to protect against short-circuit caused by accidents or vandalism. So long as a reasonable proportion of sections are serviceable, drivers would be unaffected.
Alternatively, it might be considered too complex and expensive to charge, in which case a more straightforward taxation on vehicle use in terms of miles could be used to pay for the juice.
 "This bus will run and run", New Scientist, 24 May 2003; http://www.newscientist.com/article/mg17823964.800 (subscription required).