No, it's more for the Tesla home system. They wanted solar panels to be part of the package and didn't want to have to go through another company for the high volume they're predicting. By buying Solar City, they're able to cut the fat and get the panels at cost. Even though both companies were Musk companies, they were still separate before. Lots of corporate fat that could be eliminated by combining them.
If you've followed how Musk has made SpaceX so lean and adaptive compared to other aerospace companies, this is no surprise. Bringing production for things you intend to use a lot of under your roof is a huge win in the long run. You incorporate all the existing revenue into your own and get their product at cost, all while being able to finely control quality (very important since a faulty outsourced part, one of the few SpaceX uses, caused the CRS-7 failure).