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Now that Tesla has announced detailed pricing for its Solar Roof, which the company started rolling out in August, potential customers can get an estimate on an installation using the company’s online calculator. If Tesla’s math is correct, it seems that in many cases the roof would more than pay for itself in electricity savings over the 30-year life of the warranty.

That sounds like a pretty good deal, but is it too good to be true? The answer is a complicated one, since Tesla’s calculator relies upon some important assumptions and predictions that delve deep into the economy of residential solar power in the U.S.

“It’s revolutionary in the sense of how they’re incorporating solar—the scale of it,” says David Sarkisian, a policy analyst at the North Carolina Clean Energy Technology Center, referring to the Solar Roof. “But it’s not like this suddenly changes everything about your decision on whether or not you should have solar at all. That’s still going to be informed by the same things you would look at for typical solar panels.”

Nonetheless, consumers have been quick to plunk down the $1,000 deposit, according to Tesla. The company says it's sold out of the product until early 2018.

How Tesla’s Calculator Works

Tesla eventually plans to offer four styles of tile, but currently, only two are available: smooth glass, for a contemporary look, and textured glass, which is designed to mimic the look of asphalt shingles. Each style of glass tile comes in both solar and non-solar tiles, which have different prices (the solar tiles produce energy, the non-solar tiles do not).

Tesla’s online cost calculator uses your address to determine the ratio of solar to non-solar tiles your house would need to get 100 percent of your energy from solar (the company told Consumer Reports that it doesn’t use your address for marketing purposes). The calculator deducts the upfront cost of the roof and the Powerwall battery from the cost of what you’d otherwise spend on energy over 30 years. That estimate is based on the price of electricity where you live, assuming a 2 percent increase in your utility bill each year.

The 30-year timeframe matches Tesla’s warranties both on power generation and weatherization (i.e., leaks resulting from installation) for the Solar Roof — the company claims that the tiles themselves are guaranteed for the life of your home.

Tesla’s calculator factors in a 30 percent Solar Investment Tax Credit, but not the incentives and credits that are available from various states and local utilities.

A financing option in the solar calculator uses the same ratio of solar to non-solar tiles and divides the cost, with interest, into payments over 30 years. It then compares those monthly payments to your hypothetical savings in electricity to calculate the net cost or savings per month when Solar Roof is financed.

If Tesla’s calculations are accurate, the Solar Roof may make sense for a hypothetical average house, but we wanted to see how the calculator priced a Solar Roof for real homeowners, so we put a few houses through it. Here’s what we found when we ran the numbers.

House One: New York

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We plugged the address of a CR staffer whose house is located near our headquarters in Yonkers, N.Y. According to the tool, his three-story, 2,700-square-foot Gothic Revival would need approximately 1,175 square-feet of roofing, with a recommended ratio of 70 percent solar-producing tiles to 30 percent non-solar, along with one Powerwall.

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