A reader set us a puzzle recently - how much extra range might the Pininfarina B0 gain from its rooftop solar cells in a sunny climate?

It’s tricky to answer because there are so many imponderables, not least the size of the solar array. We can’t even measure it accurately from photographs, as Pininfarina has not released dimensions for the car as a whole and we haven’t had a chance to put a tape measure across it.

So we’re guessing that the B0 will be roughly the size of the new Ford Fiesta and we will work from there...

The Fiesta is 3950mm long. This, plus some handy overhead shots of the B0, gives us a basis for our guess that the B0 boasts 1.5 square metres of solar cell, including the tiddly panel between the headlamps.

Now, let’s turn to the example of the Venturi Eclectic, which also boasts rooftop solar cells. According to Venturi, its 0.8 square-metre photovoltaic panel peaks at 70 watts, providing enough power for 2km (one and a quarter miles) per day. And we can assume that Venturi is quoting sunny-day figures on the Riviera.

Assuming that Pininfarina’s cells are similar in efficiency, this suggests that the B0 will offer roughly 1.8 times as much solar-power potential - or 130-odd watts peak power. This will probably translate to fewer than 1.8-times as many miles on the road, since the B0 goes faster than the Eclectic. But we are probably talking in the region of less than 4km or a couple of miles of benefit per day.

Put another way, a 13-amp, 240V UK domestic supply should provide 24 times the peak power of the solar cell. If the car can do 150 miles on a full charge, and it’s in bright sun for the whole time it’s charging, the maximum you could expect would be 6.25 miles of solar motoring. The figure might go up a bit because, unlike the mains cable, the solar panel can carry on providing power while the car is moving. But we strongly suspect that six miles is optimistic.

Again spurred on by the example of the Eclectic, we recommend investing in a home wind turbine. Assuming you live where there’s a bit of a breeze...