What happened

Shares of alternative-energy providers -- fuel cell stock Bloom Energy (NYSE:BE), solar panel installer Vivint Solar (NYSE:VSLR), and solar microinverter-maker Enphase Energy (NASDAQ:ENPH) -- are all on the upswing in Friday end-of-week trading. As of 1:10 p.m. EDT, Bloom stock is trading 3.9% higher, Enphase is up 5.1%, and Vivint is up 7.9%.

Why? Call it a "buy the dip" rally.

Arrow angles up on a green stock chart

Image source: Getty Images.

So what

For much of this past week, shares of alternative energy stocks like Bloom, Vivint, and Enphase have been under pressure. Investors have been avoiding the sector ever since Tesla held its Battery Day, which turned out to be a bit of a dud, depriving alt-energy plays of a hoped-for "halo" effect that might have boosted their shares.

Thus, from Tuesday through close of trading yesterday, shares of Bloom stock were down more than 13%, while Enphase and Vivint suffered more moderate 4% declines. But it seems there's only so much pessimism investors can sustain about stocks that are up more than 200% (Enphase), 300% (Bloom), and 400% (Vivint) over the past year.

Eventually, investors just want to get back in and keep riding the rally.

Now what

If you ask me, this is just what we're seeing today. There's no other news to explain why Bloom, Enphase, and Vivint are bouncing today, after all.

None of the three companies have made any substantive announcements in the last several days, such as might spark a broad rally in prices. Nor have any Wall Street analysts chimed in, said "enough is enough," and declared these stocks so cheap that they have become bargains. (Nor should they. Even after this week's declines, Enphase still costs 39 times forward earnings -- and it's the cheapest of the bunch!)

Rather, I see this as a classic "buy the dip" rally. These stocks still aren't cheap -- but they're cheaper than they were at the start of the week, and that, combined with a history of strong price gains over the past year, is enough to entice momentum investors to buy back in.