The last 15 years has seen a rapid rise in residential rooftop solar power across the US – but this is uneven, with those on higher incomes more likely to adopt solar as an energy source. In new research Aparajita Datta examines the role of state renewable policies in the spread of access of rooftop solar for low- and medium-income households. She finds that states which had previously adopted renewable energy policies – such as portfolio standards – were 3.5 times more likely to adopt solar incentives to benefit those on low and medium incomes.

Rooftop solar has grown remarkably in the US, with residential adoption growing over forty times between 2010 and 2024. However, like most technologies that have not reached widespread societal adoption, rooftop solar has an equity problem: its uptake remains skewed towards those on high incomes, and its benefits remain unevenly distributed, leaving low- and moderate-income (LMI) households behind. Over the last fifteen years, many states have introduced targeted incentives for LMI communities to address these inequities, including financing mechanisms, rebates, loans, and community solar programs. The first statewide policy adoptions began around 2010, and by 2020, 27 states had adopted at least one such targeted solar incentive. More recently implemented federal initiatives like the Inflation Reduction Act, Infrastructure Investment and Jobs Act, and Justice40 aim to expand solar access for vulnerable populations. But before energy equity and climate justice were national policy priorities, what fueled policy action at the state level?

My new research shows that previously adopted energy policies played a crucial role in the adoption and diffusion of LMI solar incentives. Scholars who think about how policies spread between places – known as policy diffusion – also consider the effects of past policy decisions on the decisions now being made – known as policy feedback. Using these theories of policy feedback and diffusion, I found that existing energy policies significantly influenced the adoption of LMI solar incentives. States with stricter electricity portfolio standards were far more likely to adopt these policies, while states being close to one another geographically had such little impact that it reduced the likelihood of adoption between states.

Residential rooftop solar adoption in the US 

Since 2010, US rooftop solar costs have dropped by over half to about $30,000 for the average home. The federal solar investment tax credit (FITC) lowers this to about $20,000, with additional savings from state and local incentives. These factors, along with market and policy shifts, have gradually shifted the median income of solar adopters from $141k to $115k (Figure 1). 

Figure 1 – Household income and income as a share of county-median income of the average solar adopter in the US

Note: The US median income was $75k for all households in 2023. Source: Lawrence Berkeley National Laboratory, 2024.

This income bracket does not represent LMI households, which make up 43 percent of Americans and are underrepresented among rooftop solar adopters—fewer than 40 out of every 100. Many cannot benefit from the FITC due to low tax liability and face challenges like limited credit, their status as renters, and high upfront costs. Targeted incentives can help overcome these barriers, reducing energy costs, burdens, and rate fluctuations while potentially increasing home values. Expanding solar access also enhances climate resilience, energy equity, affordability, and community health through reduced emissions.

State solar policies 

Rooftop solar’s equity challenge is a familiar one. Efforts to stimulate growth in any sector often result in a tradeoff with broader social equity. Moreover, decades of research on inequality and political representation have underscored that due to contrasting resources, visibility, and political behavior, governments better represent the policy preferences of the rich than less affluent citizens. This highlights the need for targeted policies to address the disparities in solar uptake.

Long before targeted solar incentives for LMI communities, and even before tax incentives were available for rooftop solar, in general, many states had adopted other policies to reduce energy costs and reduce emissions. Energy efficiency policies have encouraged the use of less energy to perform the same activity, thereby reducing costs and emissions, while energy portfolio standards have diversified states’ energy mix by establishing requirements for a percentage of electricity to be generated from eligible sources.

LMI solar incentives are policies targeted at the energy consumer. Efficiency policies and portfolio standards are targeted at the energy producer or utility. Despite affecting policy actors at the two ends of the energy value chain, the policies have shared goals and will impact the entire value chain. This is how policies, once created, affect the likelihood and form of future policies, an approach central to studying policy feedback effects. Rooted in historical institutionalism, policy feedback explores how past policy decisions shape the trajectory of new policies and create what is known as path dependence by locking in institutional arrangements and policy priorities.

Policy diffusion, another key element, examines how one jurisdiction’s policies influence others. The adoption and spread of policies depend on factors like the complexity of the policy, its compatibility with existing systems, and the specific policy area. In energy and climate contexts, policy feedback and diffusion can create opportunities for collaboration and innovation.

A state’s renewable energy policy history is more important than the policies of nearby states 

Path dependence is often seen as limiting policy innovation and responsiveness to changing citizens’ preferences. However, it can also offer benefits like policy stability, long-term growth, and increased public confidence. Energy policies are particularly prone to the status quo due to high investment costs, vested interests, public resistance, and regulatory challenges. Incremental progress can be achieved only when the process of entrenchment, the positive triggers, and the effects of self-reinforcing feedback are better understood.

Photo by Raze Solar on Unsplash

My research tracks the adoption of statewide solar incentives for low- and moderate-income (LMI) households in US states between 2010 and 2019, along with the stringency and policy scope of energy efficiency and electricity sector portfolio standards. It uses a statistical method known as event history analysis to explain how previously adopted policies and the impact of the policies adopted by neighboring states have impacted LMI solar incentives (Figure 2).

Figure 2 – Energy efficiency policies, electricity sector portfolio standards, and LMI solar incentives across US states

States that had previously adopted a combination of clean, alternative, and renewable portfolio standards, that scored as the most stringent policy choice and output for portfolio standards, were 350 percent more likely to adopt LMI solar incentives than those without such policies.

The cross-state effects of these policies were in contrast: a state with a greater share of neighboring states with portfolio standards was 90 percent less likely to adopt LMI solar incentives, indicating no evidence of geographic clustering. While cross-state feedback influences broader rooftop solar policies, LMI-focused incentives appear to be driven more by state-specific priorities than regional trends. This finding is surprising since policy diffusion has been tied to geographic clustering, where neighboring states adopt similar policies due to shared challenges and opportunities. Moreover, geographic factors like solar incidence and installer mobilization across states reinforce this trend for rooftop solar.

Interestingly, the adoption and diffusion of LMI solar incentives are independent of energy efficiency policies, both within the state and through cross-state effects.

Expanding rooftop solar access for vulnerable communities 

My findings underscore the importance of leveraging policy feedback effects to drive energy equity. States with established renewable energy policies can build on their existing frameworks to introduce targeted benefits for vulnerable communities. Encouragingly, policymakers are seeking innovative solutions beyond regional influences to address the unique needs of low- and moderate-income (LMI) communities. Simultaneously, states often purchase renewable energy certificates (RECs) from neighboring states to meet the portfolio standards targets. This reliance may inadvertently slow the adoption of LMI-specific solar incentives by reducing the stimulus to develop in-state solar capacity.

While LMI solar incentives have improved access to solar for underserved groups, many barriers still need to be addressed. For one, several states have yet to adopt these policies. Even in states with such policies, increased solar access and affordability do not automatically ensure equitable benefits for all LMI households. Challenges like energy reliability, affordability, and outreach and public education about such incentives remain significant, particularly for non-English speaking and racial minority households.

To truly balance growth and equity in rooftop solar, the profile of the average US solar adopter must shift to reflect greater inclusivity. Achieving this transformation requires a combination of robust policy innovation, targeted outreach, and consistent efforts to bridge the gap between policy intentions and household-level outcomes.

• This article is based on the paper, “Policy Feedback, Energy Equity, and Climate Justice: Can Existing Policies Improve Solar Access for Low- and Moderate-Income Communities in the United States?” in PS: Political Science & Politics. 
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• Note: This article gives the views of the author, and not the position of USAPP – American Politics and Policy, nor the London School of Economics.