CNBC's Jim Cramer sees a big difference between two similar solar energy companies, and he advised investors to heed his observations.
One company, SolarCity, remains a "must buy" for growth managers because of its association with Tesla Motors founder Elon Musk, he said. SolarCity stock exploded 389 percent in the past year, reaching an all-time high Wednesday morning.
The other, First Solar, reported weaker-than-expected earnings Tuesday and projected lower revenue forecasts, sending the stock about 10 percent lower when trading opened Wednesday.
"It's a good company, but you know what, you have to produce," Cramer said Wednesday on "Squawk on the Street."
(Read more: What's making Jim Cramer nervous about stocks)
Cramer said that's because First Solar seems on its way to becoming a more mature company compared with SolarCity, whose appeal lies in its growth potential rather than traditional metrics.
(Read more: Cramer: Don't worry, be happy, buy SolarCity)
"I find this [is] one of the signs of what you have to be careful for when you have a junior growth stock become a senior growth stock," Cramer said. "When you actually have metrics you must beat, First Solar didn't beat them, and the stock got hammered."