IN THE ten years since hybrid electric vehicles first hit the highways and byways of America, they have come to represent 2.5% of new car sales. Yet, in places like Los Angeles, the San Francisco Bay Area and Washington, DC, every other car seems to be a Toyota Prius. That is because hybrids like the Prius have sold overwhelmingly where well-heeled early adopters reside.

Expect the new generation of “Post-Prius” electrics—plug-in hybrids like the Chevrolet Volt from General Motors and those relying only on a battery such as the Nissan Leaf—to end up nosing around the same upscale neighbourhoods. With more than a dozen plug-in and pure-electric models arriving in showrooms over the next year or so, sales are expected to outstrip even those enjoyed by the Prius and other hybrids in their early days. A couple of million of the new electric vehicles could be bought by early adopters during the first few years.

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That would be a problem. Unlike the Prius and its ilk—which use their petrol engines, along with energy recovered from braking, to recharge their batteries while motoring—plug-in hybrids and pure electrics have to be recharged direct from the grid. The popular assumption is that they will be plugged into a wall socket in the garage late at night, taking advantage of cheap off-peak power. Unfortunately, things are not that simple.

For a start, the new generation of electric vehicles are not glorified golf-carts, but cleaner and more frugal alternatives to today's petrol-powered family cars. When fully charged, the Volt (to be called the Ampera in Europe) can travel 40 miles (64km) on electric power, enough for three out of four commuters in America to get to work and back without needing to burn a single drop of fuel. Beyond that range, a 1.4-litre engine kicks in to generate electricity and simultaneously propel the car and recharge its batteries.

The medium-sized hatchback Leaf can carry five adults 100 miles on a single charge. To go farther, Nissan has put its faith in a network of rapid-charging stations it is developing with partners. The Leaf is expected to cost $25,000-30,000, about the same as a comparable diesel-powered car. But the battery pack will have to be leased separately (for around $150 a month).

One thing the new plug-ins and pure electrics have in common is a beefy lithium-ion battery pack that needs a lot of heavy charging. At the very least, that involves installing 220-volt wiring in the home. Trying to recharge a modern electric car with a standard American 110-volt supply takes too long to be practical (up to 18 hours in the case of the Leaf).

Of course, if not fully charged at night it may have to be recharged during the day—when electricity rates can be up to five times more expensive. Average peak rates in America are 33 cents a kilowatt-hour compared with seven cents off-peak. Charging at the peak rate is equivalent to buying petrol at $3.63 a gallon (80 cents a litre), instead of 77 cents a gallon off-peak, reckons Southern California Edison, a utility based in the Los Angeles area. In America, peak-rate charging totally destroys any economic advantage an electric car may have.

At least the electricity companies ought to be pleased at the prospect of selling more power, day or night. In theory, recharging electric vehicles during off-peak hours should help utilities “fill the valley”—the trough in electricity demand between midnight and six in the morning, and thereby get better utilisation from their coal- or gas-fired generating stations. But, again, things are not quite as they seem. No utility wants to run its network flat out. Scheduling maintenance becomes difficult, which can lead to more frequent failures. The net result is that additional capacity has to be installed at a cost that would not otherwise be justified.

A study done a few years ago by the Pacific Northwest National Laboratory in Richland, Washington, suggested there was enough idle generating capacity in America to recharge three quarters of the country's 230m cars if they were plug-ins of one sort of another—provided they only connected to the grid during off-peak hours, and preferably in the coal-rich midwest. But the vast majority of new plug-ins will be located in a handful of urban centres on the east and west coasts, which, unlike the midwest, do not have huge reserves of cheap, coal-fired generating capacity. Nor can they import it easily from the middle of the country, given the fragile nature of the grid.

Southern California Edison has been operating a fleet of 300 electric vehicles to find out how customers will use and recharge them. Above all, it wants to make sure that a conversion to electric motoring goes smoothly, unlike a previous attempt in the mid-1990s. Back then, California thought electric cars like the Honda EV+ and the General Motors EV1 were the wave of the future, and thousands of public charging points were hurriedly installed in shopping centres, libraries and airports. But the enthusiasm collapsed when the motor industry successfully lobbied the California Air Resources Board in 2001 to get it to relax a mandate requiring 10% of new cars sold in the state to be emission-free by 2003. With no need to worry about zero-emission vehicles any more, GM and Honda promptly called in all their leased electric cars and crushed them.

This time the Californian utilities are being more circumspect. They are concerned about highly concentrated pockets of ownership and the effects of everyone deciding to recharge their electric vehicles at once—as they inevitably will do when they return home from work. The local electricity system could be easily overwhelmed, and wider swathes of the grid brought to its knees in the process. Preparing for this means beefing up local transformers as well as installing heavy-duty wiring and smart meters in homes to provide early warning of network troubles ahead. Sooner or later, those additional costs will have to be passed on to customers.

Much, of course, will depend on how quickly the new plug-ins and pure electrics become part of mainstream motoring. Generally speaking, it takes 15-20 years for a new technology to capture 10% of an established market, and a further 10-15 years for it to own 90%. That was the case when steam ships replaced clippers in the mid-19th century, and when petrol-engined taxis took over from horse-drawn cabs in the early 20th century. The same sort of lag occurred with the introduction in the 1970s of emission controls on cars. It takes years for the benefits of volume production to work their way through to the market, and for the supply chain to catch up.

If plug-in electrics follow a similar demand curve to other disruptive technologies, there could be 25m of them humming quietly around by 2025, and ten times that number by 2040. Hopefully, by then, the utilities will have learned to cope with recharging them.