Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Plug Power Inc (PLUG 10.20%) jumped 13% today after it was announced that Wal-Mart (WMT 0.26%) was the big customer who ordered fuel cell systems earlier this month.

So what: On February 10, it was announced that an order was received from a big retailer who wanted more than 1,500 GenDrive PEM fuel cell power units, as well as hydrogen and other services. Today, we found out that the customer is Wal-Mart, and they're going to deploy 1,738 GenDrive fuel cell units during the next two years. 

Now what: This is a huge order for Plug Power, but today's announcement was just details from the earlier order. The stock popped when the original order was placed, and the only real justification for the pop today is the 238 units over the initial 1,500 previously announced. This is no doubt a huge order for the fuel cell company, but I'll still take a cautious approach, and wait for more significant bottom-line results, even if that means missing out on the next pop.