Is Tesla’s Solar Roof Fair Dinkum?

Tesla’s solar roof tile announcement raises more questions than it answers.

On Saturday the 29th of October, Elon Musk made an announcement that thousands of Australians were waiting to hear.

He spent 37 seconds making that announcement.

Then he spent most of the rest of the time spruiking Tesla’s new solar tiles.

The concept of solar tiles is not new. Many companies have tried, and failed, to commercialise them. But then the same can be said for electric cars. So can Tesla make a success of their solar roof tiles?

The Tesla solar tiles are designed for American homes and so aren’t as relevant for Australians as Tesla’s home energy storage.  But building integrated solar photovoltaics – industry jargon for solar integrated into roofs, walls or windows – certainly could become big in the future.

In this post I’ll tell you everything I know about the Tesla tiles, which isn’t much.  But one thing I do know is, they sure are purdy¹.

The Feature Presentation

“How do we have a solar roof that is better than a normal roof?  That looks better, lasts longer, has better insulating effect, and the cost of roof plus electricity is less than that of a normal roof.”

Thus spake Elon Musk before the crowd inside a fake American cul-de-sac², in Universal Studios, California.  He held aloft examples of his solar tiles and told the gathering the houses around them had solar tile roofs.

As he mentioned each house, its garage door opened and a Tesla electric car rolled out.  I panicked a little when this first happened, as there was no one at the wheel and I thought Tesla was about to get some bad press from a car rolling into the crowd.  But then I remembered they can drive themselves due to a sophisticated computer auto-pilot and not at all because of demonic possession.

There were four different styles of solar glass tiles – textured, slate³, smooth, and Tuscan.

They all looked good to me.  As far as I could tell, that is.  I am no real judge of the attractiveness of American roofing material.  My taste lies in a different direction.  I love galvo or a nice bit of Colorbond.  The subtle curves of a sheet of corrugated iron…  that’s good looking roofing material as far as I’m concerned.  Of course, we all have our individual tastes.  I have a builder friend, Kal-El, whose weakness is masonite.

But good looks alone will not be enough for Tesla solar tiles to be a success.  Unfortunately, we have almost nothing but their looks to go on, as Elon gave almost no technical information.  I can tell you they face some challenges, as building integrated solar hasn’t been a success so far.

Building Integrated PV Is Not New

Sticking solar cells into roofing material is not a new thing.  Companies have been doing it for years.  While there is definitely a market for it, it is only a very small market at the moment as it has never really taken off.

The logic behind it is pretty solid and basically involves killing two birds with one stone.  Why spend time and resources installing two things, a roof and solar panels, when you can do both at the same time?  Solar panels are definitely tough enough to be roofing material.  They have to be to survive hail pounding into them, not to mention all the stones from people trying to kill two birds.  And its appearance can be preferred.

But no one has been able to get the cost of solar roofing material below that of retrofitting solar panels onto a roof.  So far building integrated solar has only gotten by on its looks and I think it will only continue to get by on only its looks for quite some time, as it faces a number of challenges.

Solar Tiles Are Less Efficient

One issue building integrated solar has is lower efficiency.  There are three main causes:

1. More gaps between solar cells.
2. Pretty coatings reduces light reaching solar cells.
3. Lack of air circulation means they get hotter than conventional panels.

Gaps

When looking at the very pretty Tesla solar tiles, it is obvious the actual solar cells take up less space per square meter than in a conventional solar panel, which means lower efficiency.  This is not necessarily a problem, provided a roof is large enough to generate the electricity desired.  And since it would be technically possible to cover the entire roof surface with solar tiles this may compensate for the efficiency loss.

Pretty Coatings

If you make solar cells look pretty by covering them with something that isn’t completely transparent, then that will block some of the light falling on them.  Tesla’s approach to this problem is to use micro-louvers which are an extremely fine grid built into the solar tiles.  When viewed from a low angle this grid is visible, which makes the tile look pretty, while from a higher angle the solar cells, or cell if it is one large one, becomes visible.

Musk says this coating only reduces efficiency by 2%.  That’s extremely impressive.  If he instead meant to say 2 percentage points then it would reduce output by around 12%, which six times less impressive⁴.

Light gets all quantumy when it hits a very fine grid, so it is possible it may only reduce efficiency by 2% when light is shining directly on it.  But I find it hard to see how it would not reduce output when sunlight is hitting it at a low angle.  Maybe they decided that since output won’t be high anyway at those times, it’s not a great loss. But if you are trying to maximise self consumption, the morning and evening sunlight can be the most valuable sunlight of all.

Unfortunately, we won’t actually know how they perform until someone actually gets their hands on one and tries it out.  I know that if I had been at the presentation I would have used a trained monkey to snag one off a roof.  The fact that no one in the crowd attempted this makes me wonder if Americans even deserve monkeys at all.

They Will Get Hot

Normal solar panels help keep the roof they are on cool and reduce air conditioner use.  This is because they shade the roof and there is a gap below them that lets air circulate and prevents hot panels from directly transferring their heat.

This air gap is pretty important.  Under extreme conditions panels without one can get up to 20 degrees hotter.  This is enough to reduce the performance of a typical panel by about 8%.  Overall, I would expect the efficiency loss from solar tiles not having an air gap to average around 4% or less.

Solar Tiles May Turn Your Roof Space Into A Greenhouse

The solar tiles are tinted glass and so significantly transparent to light.  When Musk held one up his fingers were visible through it.  This means that when installed, because of the gaps between solar cells, a considerable portion of the roof will consist of this glass letting light in which will turn the roof space into a greenhouse.   I wonder if this is the, “…better insulating effect,” Musk referred to?

They build roofs differently in America, so this greenhouse effect may be less of a problem for them. But in Australia extra ventilation could be required to stop the house turning into a sauna in the summer and further reducing solar tile performance.

Tesla Solar Tiles Appear Tough

Musk showed slow motion footage of a heavy weight smashing normal roof tiles and bouncing off a Tesla solar tile.  Since I’m pretty sure he didn’t take advantage of his rocketry side business to nip off to mars and film it in the lower gravity there, the solar tiles seem pretty tough.

With the right manufacturing techniques, glass can be made extremely tough.  If you don’t believe me, go out and smash your mobile phone with a hammer right now⁵.  Double glass solar panels consist of two glass sheets only 5mm thick in total and can withstand impacts from golf ball sized hail.  Solarwatt double glass panels only total 4mm.  The solar tiles Musk held up looked about one centimeter thick, so I could probably bang my head against them all day without damage⁶.  Of course, the actual production tiles may be thinner if they decide to save on material.

We Have No Idea How The Solar Tiles Actually Work

The arse-sack⁷ houses the Tesla solar tiles were mounted on weren’t actually real houses, but sets at Universal Studios.  And, as befitting of the location, they were fake solar tile installations and not actually wired up to provide electricity.

How they actually will be wired up, I have no idea.  Will each one operate independently or, more likely, will multiple tiles be wired into units resembling the output of a normal solar panel?  At the moment we just don’t know.

Having spent a fair portion of my life in a roof space, I can tell you we should have a much easier time wiring solar tiles together than Americans will.  We hang our tiles on an open frame while they normally put theirs on top of solid material, so how they’ll manage to wire theirs up or troubleshoot problems is beyond my comprehension.

The Fake Houses Had Massive Shading Issues

What I do know is the solar tiles on the roofs we were shown had massive shading problems.  I’m not talking about the trees around them, but the houses themselves casting shadows on the solar tiles.  Upper floors cast shadows on tiles at a lower level.  Attic windows cast shadows on tiles.  Chimneys cast shadows on tiles.  Basically it was Shadowpalooza.  Depending on how the tiles are installed, those shadows could cause huge output losses.

When describing one roof, Musk mentioned half the tiles were solar while the other half were not.  In a real installation, solar tiles may be placed on parts of a roof that get plenty of sunshine, while non-solar ones may be placed in areas that get shade.

Shading problems could also be minimized by wiring groups of tiles to microinverters so each group would operate independently and shaded groups would not affect the performance of others.  Another option would be to use DC optimization, potentially of the sort made by Maxim Integrated.

Who Will Perform The Installations?

I presume that in order to bung these solar tiles on a roof, a solar installer would have to work with roofers and develop a whole new set of roofing skills.  They would have to become some sort of super being that is more than a roofer and more than a solar installer and more than a mere fusion of the two!

But solar installers are already superheroes, so they probably won’t have too much trouble with that.

There Will Be Time Between Installation And Getting Power

When a home is being built the roof will generally go on months before it is complete.  This means there will be an extended period of time where the solar roof will be paid for but probably not producing electricity.  This is a minor problem in the vast scheme of things, but it is something to be taken into account.

Tesla Isn’t Competing On Price

Tesla is a company that became famous by taking electric cars and making them flash and fantastic and expensive.  Or at least expensive to begin with.  I am certain Tesla is attempting to do the same with building integrated photovoltaics.  Their current solar tiles are not meant for people who are happy with nice bit of $22 a square meter colorbond on their roof.  They are for people who have lots of money and like to spend it on fancy things.  And since fancy people are going to spend money on fancy things no matter what I do, I’m glad they’ll have the opportunity to spend it on some fancy solar tiles rather than a gold-plated diesel generator or a platinum proletariat powered hamster wheel.

When Can I Get Some?

Tesla says their solar tiles will start to be available in 9 months, so I guess they’re only a little bit pregnant at the moment.  But as for when they might be available in Australia, your guess is as good as mine.  If Tesla gives me a date⁸, I’ll let you know.  But getting them to meet Australian standards and be suitable for installation here is probably a hurdle Tesla won’t be willing to jump for quite some time.

Tesla Has No Advantage In Solar Tiles – And No Disadvantage

Manufacturing solar roof tiles is not an industry that faces significant barriers to entry.  That’s economist talk for the fact there are dozens of companies that could start making solar tiles right now if they wanted to.  Tesla has no special advantage in this field apart from the ability to slap their brand name on the tiles.

But brand recognition is not a bad thing to have, and while Tesla has no special advantages when it comes to solar tiles, there is no special reason why they can’t make a success of it.  But what they may not be able to do is make a huge amount of money, as there is nothing to stop competitors from moving in if it takes off and Tesla tries to keep their prices high.

Building Integrated Solar Will Catch On Eventually

As the cost of solar PV falls it will become less expensive to integrate it into roofing materials, and barring disaster, people will be richer in the future and have more money to spend on things such as making their roofs look pretty.  So I think building integrated solar will eventually catch on.  I just have no idea if Tesla will be the first company to succeed at it.

Maybe if they did a nice solar colorbond I’d be more optimistic.

The Danger Of Building Integrated Solar

One fear I have is that if building integrated solar starts to catch on it will be the only form of solar allowed in some areas, slowing the rate at which we install clean solar generating capacity.  I have nothing against making things look pretty, but I think atmospheric CO2 concentration should be falling at a rapid clip and be below what it was in 1970 before we start worrying too much about looks.