Tesla just announced the approximate cost of its solar roof: $21.85 per square foot. Consumer Reports previously estimated that the roof would have to cost no more than $24.50/sq ft to eventually pay for itself in reduced electricity cost, so Tesla comes in around 7% under budget. The problem with a generic estimate is that it doesn't account for factors such as location, climate, roof orientation, and shading, so the company also provides an online calculator that allows potential customers to estimate the energy cost savings and the net return on investment (ROI) over the solar roof's 30-year lifespan.
A couple of years ago I introduced you to a solar estimating tool created by EnergySage. That product performs a similar analysis and also provides links to companies in the area that can design and install a PV system. Obviously, Tesla's tool is specific to its own product, but I thought it would be interesting to do a side-by-side comparison of the two calculators and see how the Tesla solar roof compares to a standard PV-enhanced roof.
A Tale of Two Rooftops
Tesla's solar calculator asks the user to enter three pieces of information: address, square footage, and the number of stories. From there, it calculates the roof area, which the user can manually edit if desired. The calculator assumes a certain monthly electric bill, but that can also be modified by the user. For the purpose of this analysis, I kept the default values for electricity usage. Here's the result:
Although the calculator uses climate data to determine a location's solar capacity, it does not have a shading analysis tool. EnergySage, on the other hand, uses LiDAR data from low-flying aircraft, as well as 3D modeling tools, to perform a shading analysis. Admittedly, that data is not available for every location; it covers about 40 million US properties. Where LiDAR data is not available, EnergySage assumes no shading.
Both tools use the location to find the current electric rates, which they employ to estimate the cost savings over the life of the array. Tesla's estimator gives the bottom line, but EnergySage delivers a more robust output, showing payback period on a purchased system, and estimated savings on a leased system.
How Many Panels?
The Tesla solar calculator determined that 70% of the rooftop would be covered in PV tiles. The house I chose has its front and back roof facing east and west, but the tool doesn't take orientation into account. The total roof area is about 2000 square feet, so Tesla would cover 1400 square feet with solar tiles. EnergySage says that 750 square feet of rooftop will cover all of the home's energy needs.
Tesla shows a net ROI of $3900 over 30 years, but remember that the Tesla roof is intended for new constructions or houses that are being re-shingled, and the Tesla estimate includes the cost of a Powerwall battery, which is not included in the EnergySage report.
EnergySage estimates $22,000 over 20 years, so if we extrapolate that out to 30 years we get $33,000. To compare apples to apples, let's factor in $7000 for the cost of a battery, and $12,000 for the cost of a standard shingle job. That leaves a net return on investment of $14,000 over 30 years - more than three times the ROI of the Tesla roof.
It's not a perfect comparison, but it does show that the Tesla solar roof is a bit of an extravagance, to say the least. That's not surprising when we consider where Tesla stands in the electric vehicle market. If you just want a zero-emission vehicle that will get you where you need to go, a Chevy Bolt or Nissan Leaf will do the job at a reasonable price; if you're willing to pay two to three times more, the Tesla Model S will take you there in style. Likewise for the Tesla solar roof - it's the "Model S" of rooftop photovoltaics.
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