This year has been a good one for renewable energy. 

The world is on pace to add a record amount of capacity, especially in wind and solar, according to an International Energy Agency report out Wednesday. 

Within the next five years, the IEA is also forecasting the world will have the capacity to generate as much electricity through renewables as through fossil fuels and nuclear energy combined. 

But the capacity to generate electricity and how much electricity is actually being generated are two very different things. 

When we’re talking about energy, capacity means how much electricity a wind turbine or a solar panel would generate if it were operating at its peak 100% of the time. 

“But no technology works 100% of the time, even for gas, coal and nuclear power plants,” said Heymi Bahar, who is with the International Energy Agency and is the lead author of the report. “So obviously, renewables are dependent upon weather conditions — especially solar and onshore wind.”

Onshore wind, he said tends to generate electricity about one-third of the time and solar about a quarter or less. 

“Capacity is still a really important metric to understand how much we’re installing,” said Melissa Lott, director of research at the Center on Global Energy Policy at Columbia University. “But then we do need to ensure we’re understanding how much of the time it’s going to be running.”

The capacity that’s been added this year has also led to a record amount of renewable electricity being generated, according to the IEA.  

But the pace of expansion would have to double for the world to reach net-zero emissions by 2050. 

And in the United States? “If we’re going to have that much new capacity of wind and solar, those resources are typically further away from population centers where the electricity gets used,” said Eric Larson, senior research engineer at Princeton’s Andlinger Center for Energy and the Environment.

That will require new, long transmission lines to get that power where it needs to go. “And building transmission has historically been difficult in the U.S.,” he said.

Because, Larson added, no one seems to want it in their backyard.