There's not much to say about this, but here's my initial reaction:
It's important to understand that Tesla CEO Elon Musk is the chairman of Solar City, which is run by his cousin.
Given some takes on Tesla's cash flow situation, this gambit will allow the automaker to further diversify revenue, which is currently not limited to just car sales, but battery sales and sales of carbon credits to big polluters. Investors could be intrigued to put down more money now that it's clear Tesla is in fact much more than a one-trick pony that's focused in the risky business of manufacturing and selling autos.
On the flipside, those autos are a huge draw for the business, especially now that Tesla has taken VolksWagen's place as one of the top ten auto brand values. Considering that VW is the second largest automaker in the world, that's pretty significant.
But back to Solar City. If this $2.9 billion bid is successful, Tesla is essentially an Electric Company that leases and sells devices for power generation, storage, and consumption. That's called vertical integration, and it's a principle that has long gotten the attention of investors.