According to CR, it's not easy to weigh value of the offer, as company's online calculator "relies upon some important assumptions and predictions that delve deep into the economy of residential solar power in the U.S."

In its first analysis, CR said that costs need to be below $24.50 per square foot to match costs of conventional roofs (including the cost of Powerwall batteries, but excluding solar incentives or rebates). A 3,000-square-foot Solar Roof would then cost $73,500.

Tesla announced an estimated cost at $21.85 per square foot, which would be $65,550 for the same example house.

After using Tesla calculator for example homes in New York, Texas and California and a 30 years period, CR found out that in some cases Tesla offers a good savings opportunity, although in other cases, it doesn't look so good.  (Check out CR's breakdown for those regional examples here)

However, even if one's first calculations seem tempting, CR advises to answer some questions before placing deposit.