Over the past decade, hundreds of millions of solar panels have been installed from Sicily to Lapland, turning what was once a niche technology into Europe’s biggest source of power during the summer months. That rapid buildout is now running into a new problem: The system around it hasn’t kept up. Capacity growth is slowing, financial returns are falling and a record amount of electricity is being wasted because grids can’t handle the surge in output. With longer daylight hours, the solar season is in full swing. Capacity added over the past year has expanded the generation base and records have already been broken this spring in major markets including Germany, the U.K. and France. More are expected in the months ahead. That’s more electricity than the region can use with producers increasingly being forced to shut down plants for hours at a time on sunny days — a practice known as curtailment.…