While 160 companies around the world have committed to use “100 percent renewable energy,” that does not mean “100 percent carbon-free energy.” The difference will grow as power grids become less reliant on fossil power, according to a new Stanford study published today in Joule. Entities committed to fighting climate change can accurately measure and boost the environmental benefits of their renewable strategies, the authors write.

Current methods of estimating greenhouse gas emissions use yearly averages, even though the carbon content of electricity on the grid can vary a lot over the course of a day in some locations. By 2025, the use of yearly averages in California could overstate the greenhouse gas reductions associated with solar power by more than 50 percent when compared to hourly averages, the paper shows. One finding of this analysis is that wind power – not solar – needs to be the next wave of investments for California. Similar analyses could suggest different options like nuclear power, geothermal energy, and long-range transmission in other locations.

“To guarantee 100 percent emissions reductions from renewable energy, power consumption needs to be matched with renewable generation on an hourly basis,” said Sally Benson, co-author of the paper and co-director of the Precourt Institute for Energy.

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