"The world does not lack for auto companies," Musk said during a phone-in news conference Tuesday. "The world lacks for sustainable energy companies."

If shareholders (excluding Musk) of both Tesla Motors and SolarCity approve the deal, the solar-powered charging units could be available as early as next year, Musk said. He would not estimate how long it might take to complete the deal, which first is subject to due diligence by both companies. Musk is also chairman of SolarCity's board of directors.

"We would be the world's only vertically integrated energy company offering end-to-end clean energy products to our customers," reads Tesla Motors' offer to SolarCity. "This would start with the car you drive and the energy that you use to charge it, and would extend to how everything else in your home business is powered. With your Model S, Model X or Model 3, your solar panel system, and your Powerwall all in place, you would be able to deploy and consume energy in the most efficient and sustainable way possible, lowering your costs and minimizing your dependence on fossil fuels and the grid."

With the proposed purchase, Tesla Motors would be able to deliver a new car, a solar panel system, and its Powerwall charger all in one step, Musk said. Many Tesla owners currently count on electric recharging from power utilities using such fossil fuel as coal, which can make an electric car's upstream emissions worse than those from a conventional gasoline- or diesel-powered car.

Unlike Tesla Motors, SolarCity's stock value has struggled since its 52-week high of $61.72 per share. SolarCity's stock fell 69 cents per share Tuesday to $21.19. Asked whether Tesla seeks to buy the company because it's undervalued, Musk insisted the proposed purchase comes from "a desire to integrate products—that's really the main reason for it."