From the World Economic Forum to utility industry magazines to the US Department of Energy, in recent years there’s been a growing refrain: how batteries can enable a net-zero electricity grid. Implicit in that statement is the idea that batteries can (and should) help lower grid emissions, increase the integration of zero-emissions renewable energy sources, and support overall power sector decarbonization. Yet battery energy storage is sometimes finding itself in the hot seat for exactly the opposite reason.

Earlier this year, a University of Michigan study focused on the PJM market (the large regional transmission organization covering all or part of 13 U.S. states plus Washington, D.C.) found that batteries sometimes increased grid emissions. While the U-M study was based on older data (from 2012 to 2014), its takeaways echo concerns we’ve heard before. 

In the early 2010s, California’s Self-Generation Incentive Program (SGIP) — a major driver of the state’s behind-the-meter battery energy storage market — shifted its focus to specifically prioritize greenhouse gas reductions for the Golden State’s power grid. But then circa 2018 and 2019, analysis found that batteries were often increasing, rather than decreasing, grid emissions.

Batteries are only as clean as the electricity used to charge them

For the better part of a decade, batteries have been described as a Swiss Army knife of the power grid, capable of performing myriad functions — from customer-centric services such as backup power, peak shaving, solar self-consumption, and time-of-use energy arbitrage to grid-centric services such as frequency and voltage regulation, demand response, and mitigating renewables curtailment.

Ultimately, doing all of that involves software algorithms that dictate when a battery energy storage system charges and discharges. Those algorithms typically co-optimize around various price signals. But it’s the marginal emissions of the power grid at the times a battery is charging vs. discharging that determines whether the battery causes a net decrease (or increase) in grid emissions.

Unless energy storage considers emissions in their control approach, there’s no guarantee that they’ll help decarbonize power grids. Energy journalist David Roberts summed it up well: “It’s a mistake to deploy batteries … as though they will inevitably reduce emissions. They’re a grid tech, not a decarbonization tech,” more akin to transmission lines that can equally carry dirty or clean power, agnostic to the electricity’s generation source and the associated carbon emissions. So, too, with batteries in the absence of the right signals.

California’s battery emissions success story

To address the emissions increase caused by energy storage participating in SGIP, the rules of the program were revised with the goal of enabling the state’s participating behind-the-meter commercial and residential batteries to live up to their emissions-reducing promise. Almost immediately after the rule change, we started to see positive outcomes. A detailed impact evaluation published earlier this year by CPUC with analysis by Verdant gives a longer-term view of SGIP’s turnaround story.

Between 2018 and 2022 (the period covered by Verdant’s analysis), battery systems in California’s SGIP fully reversed course, flipping from causing a net increase in grid emissions to causing a significant net decrease in a resounding decarbonization success.

• Overall, energy storage systems went from causing additional emissions of 6.9 kg CO₂ per kWh of capacity in 2018 to delivering emissions reductions of 9.8 kg CO₂ per kWh of capacity. 
• Nonresidential systems went from bad to good, transitioning from increasing 7.8 kg CO₂ per kWh in 2018 to reducing 3.5 kg CO₂ per kWh by 2022.
• Residential systems went from good to great, improving from a modest emissions increase of 2.8 kg CO₂ per kWh in 2018 to reducing a whopping 16 kg CO₂ per kWh by 2022.

Now, energy storage has cemented its central role supporting California’s goal of achieving 100% carbon-free electricity by 2045. The state boasts more than 10 GW of installed battery capacity, and earlier this year, batteries became the single largest contributor to the state’s grid briefly during the evening peak. Grid-scale batteries charged on excess daytime solar are starting to displace natural gas power plants. And during this year’s solar eclipse, batteries charged on excess renewable energy carried California’s power sector through the temporary slump in solar PV generation.

A cautionary tale for other states

California may be the country’s most-prominent example, but it’s hardly the only US state setting combinations of both emissions-reduction / net-zero emissions targets as well as energy storage goals. For just four examples, Connecticut, Massachusetts, New Jersey, and New York — all members of the Regional Greenhouse Gas Initiative (RGGI) — each have robust energy storage targets tied to 100% clean energy and GHG reduction goals. So does Michigan.

For energy storage to help these and other states achieve their clean energy goals, it will be crucial to learn from California’s SGIP growing pains — and using a true marginal emissions GHG signal, rather than a proxy metric, to inform batteries’ duty cycles. Just look at what has transpired in Texas and the ERCOT market.

The Lone Star State has been called “the hottest grid battery market in the country.” But analysis from Tierra Climate published in June 2024 in collaboration with REsurety, Grid Status, Modo Energy, and WattTime found that 92% of batteries in ERCOT increased grid emissions in 2023. This is largely because those batteries are not co-optimizing their operation in coordination with a carbon signal like SGIP’s GHG signal. That same report found that co-optimization with a carbon signal (or a carbon price) would move these battery energy storage assets from carbon increasing to carbon decreasing.

The US energy storage market is growing fast, with record-setting capacity additions in Q1 2024 and a staggering 75 GW of cumulative new capacity forecasted to come online during the period 2024–2028. If battery energy storage is to continue living up to its promise of enabling a net-zero grid, it’s more important than ever that state policies and battery control algorithms include a marginal emissions signal as part of their intelligence under the hood.