The market hates the idea of Tesla Motors acquiring SolarCity.

After Tesla’s announcement on Tuesday of a $2.8 billion offer to acquire SolarCity, Tesla’s stock was down more than 8 percent early Wednesday morning, more than the total market capitalization of SolarCity itself. The market is ascribing a negative value to this possible acquisition.

The reaction reflects two things.

Solar City is a maker of solar energy products, basically home and business solar panels. Tesla is a maker of battery-powered cars, though some view the company’s battery-making component as its bigger future.

To Elon Musk, the chairman of SolarCity and the chief executive of Tesla, putting together these two different businesses is “blindingly obvious” and a “no-brainer.” A blog post on the Tesla website explained the reasons:

We would be the world’s only vertically integrated energy company offering end-to-end clean energy products to our customers. This would start with the car that you drive and the energy that you use to charge it, and would extend to how everything else in your home or business is powered.

In other words, the deal makes sense because people who buy Tesla’s cars also want solar power. In a combined company, they can get it in the same place.

The market is not buying it.

To investors, it is as if the Walt Disney Company bought a birthing center business to offer “end-to-end” service for its parent customers. It’s not clear that Tesla owners will really want to buy solar panels, or that if they did, it would be in sufficient number.