Why rooftop solar’s emissions math is changing

The first solar panels practically yeeted coal off the grid. But progress doesn’t simply solve our fossil-fueled problems; it reshapes them. As solar dominates and clean energy matures, success brings new complexities: managing abundance, balancing the grid, and redefining progress itself. The paradox is simple: the cleaner the grid becomes, the harder it is for each new clean megawatt to make a difference.

California offers a vivid example. The state has seen increasing solar curtailment as daytime generation pushes wholesale electricity prices to zero or even negative. This pattern, where an abundance of solar power creates its own economic and operational headwinds, is now emerging across the world’s most solar-saturated grids. Yet in California, that trend is beginning to shift. Large-scale batteries are soaking up surplus electricity, with curtailment falling as storage discharges into evening peaks, lifting average market prices and reducing reliance on fossil fuels by displacing gas peaker plants—the dirtiest units on the California grid.

Progress isn’t linear; it’s recursive. Each new solution slightly changes the nature of the problem itself.

A recent Nature Climate Change research note by John E. T. Bistline and Asa Watten of the Electric Power Research Institute (EPRI) warns that many models still use average emissions factors that no longer reflect the time-sensitive reality of modern power grids. “Emissions reductions of rooftop solar are overstated by approaches that inadequately capture substitution effects,” the title reads.

That claim refines, not rebukes, a previous Nature Climate Change study by Zhang et al., which optimistically found that rooftop PV “could provide the single largest contribution to climate mitigation in 2050.” Bistline and Watten’s follow-up adjusts the math for a more mature grid, finding that when time and regional substitution effects are included, rooftop solar’s modeled mitigation potential falls by 41% in 2035 and up to 98% by 2050 in a U.S. case study.

Source: Nature Climate Change study by Zhang et al.

The paper also highlights a deeper accounting question: if rooftop solar is added to a grid already saturated with utility-scale projects, who gets credit for the emissions reductions? The answer shifts with context. In nuclear-dominant France, hydro-powered Costa Rica, or wind-heavy Netherlands, rooftop solar displaces little fossil fuel. But in places like California—where both rooftop and utility-scale dominate daytime supply, the boundaries blur.

The researchers argue that emissions reductions are often overstated because existing modeling tools haven’t kept pace with the rapidly changing grids. They highlight four recurring issues in common analysis frameworks, noting that each region’s challenges are distinct and evolving:

Linear assumptions: Treating each new rooftop system as it offsets emissions at the same rate as the last.
Uniform emissions factors: Using global averages instead of region-specific generation mixes.
Static timing: ignoring when energy is produced—solar peaks at noon, while fossil fuels typically fill nighttime demand.
Policy effects: In advanced markets, failing to account for policy-driven growth in low-cost utility-scale solar.

The paper notes that across much of the United States, emissions data from the earlier study were “systematically higher than estimates with detailed systems modelling for nearly all scenarios and periods.”

These effects vary by geography. Regions like California and New England face steep midday “duck curves,” which limit additional emissions reductions, while others remain solar laggards where new daytime generation still offsets significant fossil output.

On Bluesky, Bistline described the paper as a “head fake”: not an attack on rooftop PV but a broader critique of emissions accounting. “These issues,” he wrote, “are key when accounting for electric sector interventions, including electrification, energy storage, data centers, and energy efficiency.” As the grid grows cleaner, each new megawatt—rooftop, community, or utility-scale—offsets less fossil fuel and more low-carbon generation.

Beyond the model

Yet even this refined focus leaves major benefits of rooftop solar outside the model’s scope, most notably its thermal performance. Rooftop systems provide both shade and insulation, limiting daytime heat gain and slowing nighttime heat loss.

Air conditioning accounts for about 19% of all residential electricity use nationwide and roughly 25% to 30% in hot states like Texas, Arizona, and Florida. Assuming rooftop solar reduces cooling demand by about 38%, as found in a University of California San Diego study, full rooftop coverage would theoretically cut total household electricity use by around 7%—before a single kilowatt-hour is consumed or exported to the grid.

A study by Vibrant Clean Energy suggests that distributed solar and storage could cut systemwide infrastructure costs by more than half a trillion dollars (inflation-adjusted, from a 2020 analysis) through 2050. Most of these savings would come from reduced transmission and distribution investments, which show up on utility bills as delivery or service charges. Ignoring physical benefits like building-level shading and insulation, along with avoided grid expansion, risks the same mistake this study aims to correct: counting what’s easy to model instead of what’s actually plugged in.

Many of the broader benefits of distributed generation like lower wholesale costs, improved local efficiency, and greater community resilience are not beyond modeling. They’re just beyond what most models choose to measure.

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