Nowhere is the greenhouse effect more noticeable than inside a car on a hot day. But the new Toyota Prius comes with new green technologies including cooling fans run by optional solar panels on the roof.

Even when the car is off and locked, these fans whir around, so when you step back into it you don’t need to crank up the power-hungry air conditioning. And the air-con system on the 2010 Prius (which confusingly was released in Japan last month and is due for release in the UK on 1 August) is more efficient to boot.

So far, so clever, but unlike the core concept of the car – the frugal hybrid drive – it is unlikely that the cost will ever be recouped by the owner. In fact, at £1,450 for the solar upgrade it is rather more than unlikely.

To sugar-coat the pill Toyota, recently trumped by Volvo to a greenest car award, has packaged it with remote-controlled air conditioning. This can be switched on a few minutes before you climb in, but the price will doubtless still leave a bitter taste for most.

For those whose environmental concerns are stronger than their bank balance, foregoing air conditioning altogether and suffering through hot days may be the only option.

Those early adopters who were first in line for a Prius years before most had heard of hybrid drives will happily pay. One day the feature will filter down to each and every car on the market, shrinking in price and becoming more powerful as it goes.

But even if the option doesn’t sell well at first, Toyota should be praised for bringing it to market –

and it is already looking into the possibility of trickle-charging the car’s battery.

The solar panels may be grabbing the most attention of all the Prius’s new features, but there are plenty more to be had on even the basic £18,370 model, in the face of increasing competition from the electric car market. It is now the most aerodynamic production car in the world, for example, allowing it to reach the same speed while using less energy.

There are also more efficient headlights, which save 17% of the power used by the old model.

These small tweaks may not make a huge difference to an individual car, driving an individual mile, but Toyota has sold more than a million, and they have collectively covered 37 billion miles.

On that sort of scale, every bit counts.