Image source: SunEdison.

Friday was Earth Day, the one day each year when the solar industry seems to get some national attention. In just a few years, solar energy has gone from being an industry struggling to survive even with high subsidies to one of the largest new sources of energy in the country, with lower costs than fossil fuels. Sometimes it's important to think about how far the industry has come.

And now, what you need to know about the week in solar.

The biggest news to hit solar in years
Ironically, the same week as Earth Day, the largest renewable-energy developer in the world declared bankruptcy. SunEdison Inc. has fallen from a $10 billion market cap to bankrupt in nine months, in a spectacular collapse driven by too much debt and bad mismanagement. The company's failure is a reminder that technological differentiation and a strong balance sheet matter more than ever in solar. SunEdison had neither, and that's a big reason it failed.

What will be important to watch in coming months is what happens to yieldcos TerraForm Power (NASDAQ:TERP) and TerraForm Global (NASDAQ:GLBL). Technically, they're independent companies, but they use SunEdison for operating and maintenance of projects as well as other services. Worse yet, TerraForm Global has already warned that a SunEdison bankruptcy could lead to accelerated debt repayments and power purchase contract changes for some projects.

Yieldcos were supposed to be a safe investment and a low-cost way to finance solar projects long-term. If both companies survive, the yieldco investment thesis will stay intact, but their bankruptcies could be worse for the industry as a whole than SunEdison's going bankrupt.

1 million and counting
Greentech Media reported last week that the U.S. recently passed 1 million solar installations. That's an amazing figure, considering how young the industry is.

More impressive is that the Solar Energy Industries Association expects that the next million installations will go up in the next two years. The pace of growth in solar is astounding, and that's one thing that makes it such an amazing opportunity for investors.

Image source: First Solar.

Earnings season is coming
This week, earnings season begins for the solar industry, with First Solar (NASDAQ:FSLR) reporting on Wednesday. Analysts are expecting $952 million in revenue and $0.88 per share in earnings, but that's not what investors should be focused on.

The key to this earnings call will be management's view on utility-scale solar growth and margins worldwide over the next few years. We've seen markets such as the U.S., Mexico, Chile, and South America commit to solar growth in just the past few months alone. Instead of the bullish but somewhat reserved view management gave at the end of 2015 about the industry after 2016, I expect much more bullishness this year.

Keep an eye out for a possible manufacturing expansion announcement, too. With a more certain industry outlook, it may be time to turn the growth jets on again.

Check for solar coverage throughout earnings season.