Elon Musk didn’t become a billionaire without brass, and this week he floated one of his most outrageous bets: an offer by his taxpayer-subsidized Tesla Motors to buy his taxpayer-subsidized SolarCity . Tesla shareholders and Wall Street analysts are howling, but didn’t they always know they were buying a business model that depended on the kindness of politicians?

The electric-car maker offered to acquire the solar panel company at a more than 20% premium over SolarCity’s previous share price in an all-stock transaction. “Tesla customers can drive clean cars and they can use our battery packs to help consume energy more efficiently,” the company said in a blog post, “but they still need access to the most sustainable energy source that’s available: the sun.”

The ostensible plan is to set up a one-stop shop so folks buying $85,000 Teslas don’t have to walk across the street to buy solar panels, among other “synergies.” Mr. Musk predicted, with his typically modest ambition, that the merger will lead to a Tesla valuation of $1 trillion, or about 34 times what it was Wednesday.

He may need one of his SpaceX rockets to get there. Tesla shares fell 10% Wednesday, or more than the $2.8 billion value of SolarCity, as investors asked why one money-losing company would be better off buying another money-losing company. SolarCity was once a darling of the green energy set, but its shares have fallen more than 50% in the past year as its political advantage ebbs.

The company’s solar-panel leasing business grew rapidly thanks to federal tax credits and state “net metering” programs that underwrite installation and let companies hawk renewable power back to the grid at high prices. New York’s Governor Andrew Cuomo wrote a taxpayer check for $750 million for a SolarCity plant in 2014.