NRG Energy Inc. is scaling back its ambitious renewable-energy plans to focus on its traditional power-generation business in hopes of bolstering its balance sheet and slumping stock.

The company on Friday announced a broad reorganization that it said would conserve $1 billion of capital in 2016, money it could use to begin reducing its almost $20 billion in long-term debt. The plan includes moving its consumer-focused solar-power business into a separate unit that will have limited access to NRG’s cash and may eventually...