Check Out Tesla's Sweet Roof
Tom Lombardo posted on October 30, 2016 | 7588 views

Elon Musk wants your roof to generate electricity.


He wants you to say to your neighbors, "Check out the sweet roof!"


Tesla and SolarCity are rolling out a solar roof that promises to be attractive, affordable, and seamlessly integrated with behind-the-meter storage and the electric grid. Musk unveiled the product at a press conference on October 28th, in a neighborhood that featured the solar roofs.


Can these roofs provide enough electricity to power a home? Well, his presentation was short on details and nearly devoid of numbers, but I'll take some educated guesses and run a few reasonable calculations to see if he's at least in the ballpark.



Integrated Home with a Solar Roof, a Battery, and an EV Charging Station
Integrated Home with a Solar Roof, a Battery, and an EV Charging Station

Affordable Solar Roof

Musk began his presentation by discussing his three-part solution to reducing fossil fuel consumption: an appealing solar roof, integrated storage, and a transition to electric vehicles. In short, it's a blend of his company's three consumer product lines. We've speculated on the solar roof, we've covered the Powerwall home battery unit, and we know about Tesla electric cars. The recent announcement unites the three technologies.


Musk noted that early electric vehicles suffered from low range and poor aesthetics, but later evolved into hot-looking, high-performance cars. He said, "Something similar needs to happen to solar. We really need to make solar panels as appealing as electric cars have become." The goal is to produce a residential roof that looks nicer, lasts longer, insulates better, and generates enough electricity to run the house. Oh, and its installed cost must be less than the price of a normal roof plus the cost of electricity.


How much will the system cost? He didn't mention that. Details, details...


Integrated PV


Pop quiz: Which of the following roof tiles have solar cells in them?

Tesla Roof Styles - most expensive (left) to most affordable (right)
Tesla Roof Styles - most expensive (left) to most affordable (right)

Answer: ALL of them. The solar roof is made of photovoltaic panels cleverly disguised as high-end roof tiles. How do they accomplish this feat? Micro-louvers.

How To Disguise and Protect a Solar Cell
How To Disguise and Protect a Solar Cell

A thin layer of micro-louvers presents one look to the street and another view to the sun. Streetside, the observer sees the attractive, colorful louvers; from above, the louvers are edge-on, exposing the high-efficiency PV cell to sunlight.


Two Views of the Same Roof: Street View (left) and Sun's Eye View (right)
Two Views of the Same Roof: Street View (left) and Sun's Eye View (right)

Although the micro-louvers improve the aesthetics of the solar roof, they also decrease its performance a little. PV cells work best in direct sunlight, but they also produce electricity under diffuse and reflected light. The louvers will reduce the amount of reflected light that reaches the cells. How much that affects the panel's performance depends on several factors, including the albedo (reflectivity) of the surroundings, the sun's position relative to the roof, and the tilt of the PV array. Let's say, for the sake of argument, that the louvers will reduce the output by ten percent. SolarCity produces PV panels with 22% module-level efficiency. Putting micro-louvers in front could drop that to around 20% - not really a big deal for tiles on the south, east, and west sides of the roof. (You might remember the Sistine Solar roofs, whose solar panels have a thin coating which allows most light to pass through, but also presents a pleasing image to the viewer. Their panels are about 16% efficient.) Tiles on the north side, which can generate electricity from diffuse or reflected light, will suffer more, due to their angle away from the sun. But the fact that the entire roof is covered with PV cells compensates for the loss of power due to the louvers.  


Of course, how much electricity the roof can generate will depend on what direction the PV tiles are facing, shading conditions, location, and other factors. On the conservative side, I estimate that a 2000 square foot home (186 square meters) could easily accommodate an 18 kW rooftop solar array which, assuming a significant unshaded portion faces south or southwest, is capable of producing 99 kWh of energy on a summer day in the northern US, and 63 kWh/day in the winter. The average US home currently uses about 30 kWh of electricity each day. Of course, there will be consecutive cloudy days where the system produces less, which is why the system is designed with storage and grid-tie capability.


What about Partial Shading?

A little shade is a very bad thing when it comes to rooftop solar. A panel that's just 10% shaded can reduce the total array output by 80% or more, so PV designers perform a shading analysis to be sure the panels aren't shaded during peak generating hours. Obviously, an entire roof covered in solar tiles will have some of them shaded at any given time. One way to compensate for that is the use of microinverters, which maximize the power delivered by each module and allow the modules to run independently of one another. Microinverters are ideal for grid-tied systems that don't have battery storage since they convert the panel's DC output to grid-compatible AC right at the panel itself. But batteries are DC devices, so microinverters are out of the question. Instead, I suspect they're using either a DC optimizer, which handles maximum power point tracking at the panel level, or some other kind of intelligent electronics built into the panel. More mystery...



Ain't That Tough Enough?

Will the solar roof stand up to hailstorms? Thanks to the tempered glass exterior, here's how they compare to traditional roof tiles:

Tempered Glass PV Tiles Are Tougher than Most Roof Tiles (Image: SolarCity)
Tempered Glass PV Tiles Are Tougher than Most Roof Tiles (Image: SolarCity)

Storage

The solar roof is designed to integrate with Tesla's Powerwall 2, a $5500 behind-the-meter storage device that holds 14 kWh of energy and delivers up to 7 kW of peak power. Based on the estimates above, it should be easy for the system to keep the battery charged AND sell some power to the grid during the summer, while it's likely to be necessary to buy some grid power in the winter. (This is based on a northern US location. Southern regions will get even better performance.)



Utilities: Have No Fear

Holy Solar, Batman! Is this the end of electric utilities?


Not so fast, Robin. You see, in order to electrify transportation and heating, the demand for electricity is expected to triple. We'll still need the grid - hopefully with power generated from renewable sources - to supplement our rooftop generating capacity, and to provide power to apartment buildings, skyscrapers, and industrial users. Musk sees the electric utilities as partners, not enemies, which is why he's selling grid-level storage as well.



Here's the video of the unveiling:



There's no doubt that the technology is feasible - the only question is whether it's affordable. Musk mentioned the history of electric vehicles regarding performance and aesthetics, but he didn't discuss the fact that the first EVs that had decent range and looks were also very pricey. Now that the technology has matured, EVs have become more cost-effective than gasoline powered cars. "Regular" solar panels have a payback period of 10 to 25 years, depending on location and electric rates, so they're already at or near grid parity. How much will consumers be willing to spend for a "sweet" roof?



Images and video courtesy of Tesla Motors


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