Tesla is lining up a big new profit driver as California prepares to cover a large chunk of the cost of its electric semi-truck — good news for Tesla shareholders, less so for taxpayers.

• The Tesla semi is set to launch in Europe, but the real money story is California.
• State incentives could reach $120,000 per truck, close to half the base price.
• Running an electric semi costs about half as much per mile as diesel, worth roughly $36,000 a year in extra profit for the operator.
• Government subsidies on Tesla products have historically pushed the company's profit margins sharply higher.

Outlook: If California's incentives land as expected, the semi becomes a high-margin business for Tesla and a fresh tailwind for the stock.