In recent years, scores of corporations have made renewable energy pledges. RE100, a corporate initiative aimed at getting companies to commit to purchasing all their electricity from renewables, counts 176 companies among its members, including Ikea, Coca-Cola Co. and Apple Inc.

But in many cases, firms aren’t actually consuming renewable energy. They’re signing power purchase contracts with renewable developers. Electricity from the developments goes into the grid. Companies get renewable energy credits, which they use to offset emissions from their power consumption.

The dynamic has led to a debate in corporate climate circles in recent years over how to match renewable generation with companies’ electricity consumption.

Enter a new study from Stanford University’s Precourt Institute for Energy. The study’s most important finding: A 100% renewable energy commitment does not mean firms’ electricity is completely carbon-free.

“To guarantee 100% 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 the institute’s co-director.

The conclusion owes itself in large part to the nature of renewable production. Solar generation, for instance, surges in the middle of the day, reducing the carbon intensity of the grid. But during nighttime hours, the carbon intensity of electricity generation increases, especially if it is not complemented by another carbon-free resource like wind.

The dynamic can prompt companies to overestimate the carbon benefits of their renewable energy supply, the Stanford researchers concluded. They studied the case of a theoretical consumer with 1 megawatt of constant demand in California, a state with significant and ever-growing solar generation. To achieve 100% renewable energy, that consumer would need to buy either 3.16 MW of wind or 3.6 MW of solar.

Measuring the emissions associated with that energy on an annual basis would prompt the consumer to overestimate emissions by 50% in 2025, the researchers concluded. An hourly count of renewable energy production would produce a much more accurate picture of emissions reduction.

“If we want to go beyond slogans, if what we care about is carbon in the atmosphere, then we need to measure it,” said Jacques de Chalendar, a Stanford doctoral student and the study’s lead author. “One hundred percent renewable was great last decade. It’s now about setting new goals for ourselves.”

The dynamic is not limited to solar. The same situation could unfold in the United Kingdom, where offshore wind accounts for a growing share of electric generation.

The researchers conclude that companies interested in emission reductions will ultimately need to use energy storage and schedule their power consumption to times of the day when wind or solar is pumping out electrons.

Microsoft Corp. hailed the study. In recent years, Microsoft has begun to push beyond a traditional 100% renewable target in an attempt to match renewable generation with its electricity consumption.

The report speaks to a “critical need,” said Elizabeth Willmott, carbon lead on the technology company’s sustainability team. Measuring carbon emissions on an annual basis “is like trying to set a daily budget with only a single year-end bank statement,” she said.

She called the data a “prerequisite for behavior change.”

“We envision these solutions rewarding and enabling a shift in demand from times of grid congestion to times of high renewable power and optimizing energy consumption to times of lowest marginal carbon emissions—timing the charging of your EV to when your local solar is producing at midday rather than the middle of the night when solar is low, for example,” Willmott said.

Call it a foundation for a new corporate climate initiative.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news