How much people want solar panels depends upon who is going to pay for the solar panels CC0,

Anyone with a brain could see that they were going to be before anyone started. Whatever the benefits of solar electricity generation they’re going to be lessened y having the panels flat, covered by enough glass to carry the weight of traffic and shaded by those vehicles as they pass. The least surprising thing is that this is also as it has turned out, solar roads are a ludicrous waste of money. From The Conversation:

Solar panels replaced tarmac on a motorway – here are the results

Four years ago a viral campaign wooed the world with a promise of fighting climate change and jump-starting the economy by replacing tarmac on the world’s roads with solar panels. The bold idea has undergone some road testing since then. The first results from preliminary studies have recently come out, and they’re a bit underwhelming.

A solar panel lying under a road is at a number of disadvantages. As it’s not at the optimum tilt angle, it’s going to produce less power and it’s going to be more prone to shading, which is a problem as shade over just 5% of the surface of a panel can reduce power generation by 50%.

The panels are also likely to be covered by dirt and dust, and would need far thicker glass than conventional panels to withstand the weight of traffic, which will further limit the light they absorb.

Read more: Solar freakin’ roadways? Why the future of this technology may not be so bright

Unable to benefit from air circulation, its inevitable these panels will heat up more than a rooftop solar panel too. For every 1°C over optimum temperature you lose 0.5% of energy efficiency.

As a result a significant drop in performance for a solar road, compared to rooftop solar panels, has to be expected. The question is by how much and what is the economic cost?

The road test results are in

One of the first solar roads to be installed is in Tourouvre-au-Perche, France. This has a maximum power output of 420 kWs, covers 2,800 m² and costed €5m to install. This implies a cost of €11,905 (£10,624) per installed kW.

While the road is supposed to generate 800 kilowatt hours per day (kWh/day), some recently released data indicates a yield closer to 409 kWh/day, or 150,000 kWh/yr. For an idea of how much this is, the average UK home uses around 10 kWh/day. The road’s capacity factor – which measures the efficiency of the technology by dividing its average power output by its potential maximum power output – is just 4%.

The solar road is unveiled in Tourouvre au Perche. Christophe Petit Tesson/EPA

In contrast, the Cestas solar plant near Bordeaux, which features rows of solar panels carefully angled towards the sun, has a maximum power output of 300,000 kWs and a capacity factor of 14%. And at a cost of €360m (£321m), or €1,200 (£1,070) per installed kW, one-tenth the cost of our solar roadway, it generates three times more power.

In America, a company called Solar Roadways has developed a smart highway with solar panels, including sensors and LED lights to display traffic warnings about any upcoming hazards, such as a deer. It also has heating pads to melt snow in winter.

Several of their SR3 panels have been installed in a small section of pavement in Sandypoint, Idaho. This is 13.9 m² in area, with an installed capacity of 1.529 KWs. The installation cost is given as $48,734 (about £37,482), which implies a cost per installed kW of €27,500 (£24,542), more than 20 times higher than the Cestas powerplant.

Solar Roadway’s own estimates are that the LED lights would consume 106 MWh per lane mile, with the panels generating 415 MWh – so more than 25% of the useful power is consumed by the LEDs. This would reduce performance even further. The heating plates are also quoted as drawing 2.28 MW per lane mile, so running them for just six days would cancel out any net gain from the solar panels.

And this is before we look at the actual data from the Sandypoint installation, which generated 52.397 kWhs in 6 months, or 104.8 kWhs over a year. From this we can estimate a capacity factor of just 0.782%, which is 20 times less efficient than the Cestas power plant.

That said, it should be pointed out that this panel is in a town square. If there is one thing we can conclude, it’s that a section of pavement surrounded by buildings in a snowy northern town is not the best place to locate a solar installation. However, perhaps there’s a bigger point – solar roads on city streets are just not a great idea.

The driveway prototype which inspired Solar Roadways. Dan Walden/Wikimedia CommonsCC BY-SA

Running out of road

Roads don’t actually represent as large an area as we assume. The UK department of transport gives a breakdown of the length of the UK’s different road types.

Assuming we can clad these in solar panels, four lanes of every motorway, two lanes on the A & B roads and half a lane for C & U roads (a lot are single track roads and just won’t be suitable) we come up with a surface area of 2 billion m².

Which sounds like a lot, until you realise that buildings in the UK’s urban areas occupy an area of 17.6 billion m². So just covering a fraction of the UK’s rooftops with solar panels would immediately yield more power than putting them on roads. That’s quite apart from the benefits that a more elevated position would yield for greater power generation.

All of this suggests that only a small fraction of the road network would actually be suitable. And, given the relatively small size of the road network, solar roads could only ever become a niche source of power and never the shortcut to our future energy supply.

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One part of the analysis is obsolete: It used to be that shading of one panel in an array reduced the output of the entire array, which led to shocking results on the top of a garage with a nearby tree. But modern panels include electronics that feed power directly to AC mains and don’t have this problem.

Nevertheless, a “viral” campaign to counter alleged catastrophic man-made Global Warming or Cooling cannot be expected to lead to the best engineering. As for wasteful deficit spending to “jump-start” the economy: We tried that. Obama Economic Malaise.

Quentin Vole
Quentin Vole

Even if you’re a true believer that CO2 is the Devil’s flatulence, PV panels installed at temperate latitudes (the ones in the article are west of Paris) are unlikely, during a realistic working lifetime, to generate more energy than was consumed during their manufacture – and that’s without all the additional negative factors that Tim mentions, when they’re set into a road surface.

So they do nothing to reduce global CO2 emissions. (But still good for harvesting government subsidies.)


The milk are in?
The sand are in?
The rice are in?
The beer are in?
Mass nouns are singular.


This one is raw Latin. The datum is in; the data are in.


The solar panels that people should be using is solar water heating. No fancy schmancy rare minerals and feed-in tarrifs, just a couple of hundred feet of tubing mounted flush with the underside of the slates, best installed when you happen to be doing some other work on the roof anyway. I was going to do that when I replaced my roof in the 1990s, having expirenced the near-passing-out heat inside the roof when rewiring the house, but the company I approached had gone bust. Instead, I just put a couple of rainwater storage tanks fed from the roof to… Read more »


And as suggested by your fraction, it doesn’t have to raise it to bathwater levels; any heating represents a savings.

Likewise, I have off-the-shelf panels lighting the deck and inside the woodshed, but refuse to put a system on my roof with the industry in flux and the products not yet perfected and commoditized — just as it was a mistake to dabble in house LEDs before the current generation that screws into existing sockets.