Every now and then I get asked if there is any benefit to installing solar panels on their electric cars to recharge them while they perform their most common task: sitting around not moving.

The short answer is no, it’s not typically worth it.

The long answer is because of a lack of efficiency in the solar panels, and the fact that as efficient as they are compared to gasoline/diesel vehicles, electric vehicles still use a reasonable chunk of electricity.

For example, a typical home-converted electric car uses around 300 watt-hours (Wh) to travel 1 mile. A watt-hour is a measurement for electricity used over a period of time. It’s the only way to measure electricity as if it were a unit. For this reason you could imagine a watt-hour is like a spoonful of electricity.

So, now that we know it takes your electric car around 300 Wh to move a mile, let’s figure out how much a big fat solar panel of the roof can generate. Let’s start with one of those 4-foot (1.2 meter) monsters.

A really good one might put out around 150 watts in bright sunshine. Therefore over the course of an hour that works out to be 150 watt-hours, or 150 watts per hour. With me so far?

So for your car to move a mile – which uses 300 Wh – you’ll need to have it parked in constant sunshine for a minimum of two hours. This is because 150 Wh x 2 = 300 Wh.

Or in really simple English, 1 hour in the sun = 150 spoonfuls of electricity generated; 2 hours in the sun = 300 spoonfuls of electricity generated; and the car uses 300 spoonfuls per mile.

Therefore in order to drive 5 miles your single 150 W solar panel would have to sit in the sun for ten long hours. Maybe in Australia or California this could be helpful, but in some locations you’d be lucky to get 10 hours a month.

This is why the solar-powered cars you may have seen racing across the Australian Outback are all very light, very flat, and covered from tip-to-tail in expensive solar panels. Yet even with all those panels, and all that “Downunder” sunshine, they only reach 62 mph (100 km/h) occasionally.

Car makers are still looking into solar panels as a helping hand however, as they can prove useful in sunny locations to help with the air conditioning and stereo etc, but they’re not efficient enough to recharge an electric car’s 16,000 Wh battery pack – unless you can afford to wait a month in the sun!

I hope that helps to explain how much power a typical solar panel puts out for it’s size, and why many electric car converters aren’t too keen on spending such a large sum of money for such a small overall result.
Why not spend that $800 on electricity instead – that will pay for more than 2 years of electricity used in a typical electric car!