NextEra Energy Inc. plans to invest $100 million in energy-storage systems over the next year to back up its fleet of solar and wind energy plants. NextEra is the world’s biggest producer of wind and solar power, and will use the battery systems to manage energy fluctuations caused by cloudy weather and lulls in the wind, Jim Robo, chief executive officer of the Juno Beach, Florida-based company, said Tuesday at an analyst conference. Power companies are showing more interest in storage, especially batteries, which are becoming cheaper. The technology offers the potential to make intermittent power sources more reliable and more affordable, Robo said.