While following the progress of Tesla solar roof installations in the San Francisco Bay Area, we discovered that some of the shingles were being made in China.

We also caught this product label which detailed the electrical specifications of Tesla solar roof tile model SR 60T-1, which provides enough detail to calculate the efficiency of the tile. (This product is labeled as made in the U.S.A.)

In order to figure out the efficiency of a solar module you have to do a little math. First, you multiply the length and width of the full solar modules including the non-power producing frame to get the area of the product. Second, you divide the module’s area by 1,000 since solar modules are rated based on standard test conditions of 1,000W/m2. Lastly, you divide the module’s rated wattage output (24 W in this case) by the module’s area which should provide the efficiency value.

In the case of the Tesla solar module specifications, we’re given dimensions of 1140mm x 430mm. Based upon these values on the label (which do not represent the exposed area of the final installed product), the efficiency of the product is 4.9%. The first modern solar cell invented in Bell Labs in 1954 had an efficiency of 4%.

Here is where we speculate a bit based upon limited data and attempt to apply some common sense. Feel free to check our math.

If we assume that the plastic pieces that protrude from the edges of the module are included in the sizing, then we can knock off roughly 10% of the width and 33% of the height – which results in an efficiency of 8.1%. Better, but still at the efficiency levels of the late 1950s.

Taking into account the overlap on how the tiles are installed on the roof — we can make another speculative size adjustment. All of the installations show that the solar modules overlay other modules to one side and below.

If we assume another 20% off of the height, and 10% off of the width — our efficiency hits 10%. Still a low value compared to the average residential solar module efficiency of around 18%.

However, reports have suggested that the solar tile installations meet 100% of the electricity usage of the customers — so, does efficiency actually matter?

It matters up to the point where it meets the needs of electricity users.

If Tesla’s solar roof is able to meet home power needs because customers are able to install the tiles on a larger proportion of the homeowner’s roof — then the importance of efficiency falls away compared to the effectiveness of the product.

Tesla roof installations that we’ve monitored are taking ten days to two weeks to complete. So, whether this product can be profitable for Tesla or roofers or provide a return-on-investment for homeowners is another story.