NRG chief executive David Crane, who has led the company's drive toward renewable energy since 2009, is impressed with the progress across the company's expansive solar portfolio.

Its three large solar projects are proceeding on or ahead of schedule. NRG reports more than 260MW of solar in operation, and more than 850MW under construction.

Because of that faster-than-expected progress, NRG now expects its solar assets to contribute $70m-75m of earnings before interest, taxes, depreciation and amortization (EBITDA) this year, up about $5m from expectations three months ago.