Wind turbines turn behind rows of solar panels in Avignonet-Lauragais, in the Midi-Pyrenees in France. Fred Lancelot/REUTERS Europe gets so much of its energy from renewables that many leading countries are now facing a new problem: updating their energy grids to handle it all.

Last year, developing countries — including China, India, and Mexico — invested more in renewable energy than developed ones, including the US and those in Europe.

This is according to a UN-backed report released on March 24.

Part of the reason for this switch was because of enormous investment in China. More than a third of all the renewables investments around the world in 2015 occurred there.

But it's also because clean-energy spending slowed in Europe — down 21% from $62 billion in 2014 to $48.8 billion in 2015, according to the report.

Ulf Moslener, professor for sustainable-energy finance at the Frankfurt School and coauthor of the report, explained why Europe's investments are declining.

"These economies already have a significant part of renewables, and are now struggling with adapting their energy systems to these new technologies," he said at a press conference. "Storage will play a large role in fixing that."

The Amrumbank West offshore wind park near the island of Amrum, Germany. Morris Mac Matzen/Reuters

Since renewable energies like wind and solar can't provide energy 24/7, battery storage captures it for when the wind's not blowing and the sun's not shining.

Countries need to update their electricity grids and add more storage before they will be able to fully capture all the renewable energy they can now generate.

"We're really at a very interesting juncture ... the leading markets — Germany, Denmark, Spain, the UK — are going to do some pretty substantial structural changes," Michael Liebreich, chairman of the advisory board at Bloomberg New Energy Finance and coauthor of the report, said at the press conference. "I'm not surprised they've slowed down investments while they do that."