What: A day after hitting a 52-week low, shares of Plug Power Inc (PLUG -3.43%) rose as much as 17% in early trading today after the company provided some second-quarter operating figures.

So what: GenDrive shipments more than doubled, going from 419 units in the first quarter to 888 units in the second quarter. Management said that would lead to record revenue, although they didn't actually disclose any figures.  

Maybe more important was management's confirmation that it still expects to generate sales of $100 million in 2015 on sales of 3,300 GenDrive units and 15 GenFuel hydrogen systems. Bookings are expected to be over $200 million.

Now what: Without actual sales figures, it's hard to know how good the quarter actually was, and investors will have to wait until the earnings report in early August to hear more. While growth is important, investors should also look at net income as a gauge of Plug Power's progress. Analysts aren't expecting a profit from the company this year or next, so an upside surprise would be welcome progress for investors.

I'm leery of this stock given its $420 million market cap and just $100 million in expected revenue this year, but if the bottom line improves and bookings remain strong, there will be signs of value emerging in this stock.