Gregory Milligan

A common question is why don’t EVs have solar panels on the roof? Wouldn’t this be sensible? We could drive to work then have the car charge itself up while we work for the drive home…

Like many ideas, it’s nice in theory, but it doesn’t work in practice.

Here’s why…

We will take the Nissan Leaf as an example. With its dimensions of just under 4.5m by just under 1.8m it theoretically has a footprint of 8 square metres, although with windscreens etc we can probably estimate 5 m2 of usable space on which a solar panel could be put.

Solar panelsSolar panels generate between 100-200 Watts per m2, depending on the type and expense and also on the latitude where they are. The flexible easily moulded panels that would be suitable for the roof of a car are the amorphous panels which are at the lower end of the efficiency range. We will assume 120W/m2 for this exercise with a yotal maximum output of 600W.

The sun doesn’t shine strongly all day, it varies with time of day, clouds, etc. A rule of thumb for estimating solar output is 4-5 “sun-hours” per day, which means that over a whole day the panels will usually generate the same amount of power as if they had been in full sun at maximum output for 4-5 hours.

So, in ideal condition of sun, with no shade and no horrible rainy overcast day, a Nissan Leaf covered in solar panels would be producing around 2400 Wh per day. A Nissan Leaf battery pack holds 24,000 Wh which would mean that you need to park your Leaf in the sun for 10 days to charge it from empty to full…..

Ok, we are somewhat simplifying this, you could cover the entire body in solar panels and there would be some generation coming from the sides and rear as well as the roof…but there are other factors too, finding a park in full sunlight is well-nigh impossible for most of us, and any shade will kill solar panel output very quickly. But…even if you could double the amount of energy produced, by using the most efficient panels available, plastering the entire car with them, and parking on top of a sunny hill…you would still only generate enough power in a full day to go an extra 14 kilometres…

For the several thousands of dollars that the solar panels would cost to add to your vehicle, you could add more batteries that would give you 50 kilometres of range…so putting solar panels on your EV simply doesn’t stack up.

Those solar powered race cars you may have seen on television racing across the Australian desert are high tech super expensive vehicles designed for one purpose only, to carry one person across the desert as fast as possible on only solar panels. They gain their top speed by being extremely aerodynamic, for which the sole occupant must drive in a reclining position to reduce the area exposed to wind resistance. The tyres are skinny and hard, like bicycle wheels, for low rolling resistance. They are extremely lightweight and have tiny motors. In fact they are very much like a glorified very expensive high tech recumbent tricycle with an electric motor and a bunch of panels on them…..not a very practical family transporter or grocery getter!

You may see solar panels on some EVs…some Nissan Leafs for example have a small solar panel on the roof spoiler. This powers a ventilation fan which extracts hot air from the car when it is parked in the sun, which means when you return to your car it isn’t so hot and the air conditioning doesn’t need to work so hard to cool you down. The interior of a car in sunlight can reach 50 degrees or more on a 30 degree day, so this makes a huge difference when you are relying on battery power to cool the cabin down!