Every day, when the sun comes out, solar panels send electricity to the grid, displacing coal and gas.

But swapping out fossil power is rarely a simple, one-for-one process. Coal plants can need hours to throttle up and down, so the emissions impacts of solar don’t appear immediately. And when surplus electrons from sunny areas are transmitted to neighboring grids, they displace fossil generation there as well.

Ignoring those variables can exaggerate or underestimate the true climate benefit of solar. A clear accounting of these impacts helps utilities, regulators, and investors decide where and how fast to deploy solar and storage.

My team and I just published a study in Science Advances where we highlight the real power of solar energy to fight climate change, even as the policy landscape changes.

By crunching five years of nationwide grid data, we show that solar’s climate punch is bigger and quicker than many policy models assume, offering a concrete roadmap for states scrambling to meet net zero goals.

Ramping up solar generation by 15% across the United States could slash annual carbon dioxide (CO₂) emissions from electricity plants by 8.5 million metric tons (MMT), we find. That’s about 12% of the 2042 reduction targets set last year by the Environmental Protection Agency.

Elephant in the room

The Trump administration has proposed to rescind those targets and revise other policies that encourage clean energy.

The One Big Beautiful Bill Act signed by the president last month greatly curtails support for clean energy production, including solar. The Interior Department is considering new layers of political review for wind and solar development, adding extra hurdles that threaten clean energy targets. And, on July 29, EPA administrator Lee Zeldin said the Trump White House would seek to revoke the EPA’s 2009 “endangerment finding,” the determination that greenhouse gases including CO₂ endanger public health and welfare. Each of these moves would result in higher CO₂ emissions.

Not all solar is equal

Our study provides a more detailed view of system-wide effects from expanding solar than earlier work, identifying where solar adoption will deliver the greatest benefits.

We first gathered hour-by-hour data on electricity demand, solar power output, and the amount of CO₂ released by power plants in each of 13 U.S. grid regions. Then we built a statistical model to check how a one-hour bump in solar generation affects CO₂ reductions not only in that same hour (the immediate effect) but also in each of the next 12 hours (the delayed effects).

Adding more solar insunny states like California, Texas, and Florida results in large emission drops. In California a 15 percent boost in solar output at noon corresponded to a 147-metric-ton decrease in CO₂ within that hour and a further 16-metric-ton drop eight hours later.

The picture is more mixed as you travel north.

Compare New England and the mid-Atlantic states. New England has less sunshine. In addition, its gas-dominated power fleet operates more efficiently than the coal plants common in the mid-Atlantic. So every added solar panel in New England displaces less CO₂ than one in the relatively sunnier mid-Atlantic.

The benefits are amplified in regions with transmission lines capable of exporting surplus solar – or batteries to hold those electrons for use in the evening.

We estimate that expanding California’s solar capacity by 15% would lower daily CO₂ emissions by around 913 metric tons in the Northwest and about 1,942 metric tons in the Southwest thanks to existing transmission networks.

Smart siting amplifies gains

The spillover impacts of solar adoption on neighboring regions underscore the importance of coordinated clean-energy efforts.

Our results give policymakers and businesses the region-specific insights they need to direct solar investments – and supporting policies – toward locations that deliver the biggest CO₂ reductions.

While environmental policies may swing with politics, scaling up solar energy offers a robust, measurable way to cut carbon pollution. Every ton helps move the U.S. closer to meeting its climate goals and to protecting public health from the fine particulate matter (PM2.5) emitted by burning fossil fuels.

All perspectives expressed in the Harvard Climate Blog are those of the authors and not of Harvard University or the Salata Institute for Climate and Sustainability. Any errors are the authors’ own. The Harvard Climate Blog is edited by an interdisciplinary team of Harvard faculty.