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Can Low-and-Moderate Income Households Play a Role in Realizing U.S. Rooftop Solar Technical Potential?

September 12, 2018 by Kyle Flanegin

Low-to-moderate (LMI) households, defined as having income levels lower than 80% of the area median income (AMI), make up 43% of the population in the United States. LMI households typically have the highest energy burden—the ratio of energy expenditures to overall household income—of all income groups and could benefit from participating in the distributed photovoltaics (DPV) market. However, LMI populations historically have not adopted DPV as widely as middle-to-high income groups for a variety of reasons, including the lack of access to capital, insufficient tax burden, and distorted price signals.

Until recently, the technical potential—or the maximum generation potential from DPV systems—for LMI residential rooftops was not well understood. However, a recent NREL report, “Rooftop Solar Technical Potential for Low-to-Moderate Income Households in the United States” explores this issue by evaluating DPV potential on residential rooftops by income levels. The report addresses several key questions about the DPV technical potential of LMI residential rooftops, including:

  1. What is the quantity and spatial distribution of rooftop technical potential, stratified by income, building type, and tenure?
  2. Among LMI households, what is the feasibility of achieving parity in solar access across income groups?
  3. To what extent might non-profits oversize systems on their roofs to export excess generation locally to LMI households?

To answer these questions, the authors leveraged a LiDAR dataset produced by NREL in 2016 in combination with U.S. Census Bureau socio-demographic and building stock data at the tract-level to study rooftop potential in 128 metro regions. The authors then extrapolated this data to areas that were not covered in the original dataset using statistical techniques to quantify nation-wide technical rooftop potential by income, building type, and tenure.

Technical Potential by Income, Building Type, and Tenure

To answer the first question regarding rooftop solar technical potential by income, building type, and tenure, the report found that 1,000 terawatt-hours per year of DPV generation potential exists on all residential building stock in the United States. Partitioned by income, 42% of overall technical potential exists on LMI-owned or LMI renter-occupied buildings. By building type, single-family homes (including both owned and rented dwellings) account for 68% of the technical potential, while multi-family buildings represent 32% of the technical potential. To date, the majority of rooftop DPV deployment has occurred on single-family, owner-occupied homes. Among LMI buildings specifically, single-family, owner-occupied homes still hold the largest single modality of DPV technical potential across different combinations of housing structure and tenure type; however, 60% of the rooftop DPV potential for LMI buildings is located on multi-family and renter-occupied buildings (see Table 1).

Table 1. Distribution of Residential PV Rooftop Technical Potential by Income Group
Income Group Households (millions) Suitable Buildings (millions) Suitable Module Area (millions of m2) Capacity Potential (GWDC) Annual Generation Potential (TWh/year)
LMI Very Low
(0-30% AMI)
19.5 9.4 794.4 127.1 160.8
Low
(30-50% AMI)
11.5 5.7 472.8 75.6 95.3
Moderate
(50-80% AMI)
18.8 10.4 792.0 126.7 159.8
Non-LMI Middle
(80-120% AMI)
21.1 12.3 900.4 144.1 180.8
High
(> 120% AMI)
46.0 29.4 2003.3 320.5 403.1
All LMI Buildings 49.8 25.5 2059.2 329.4 415.9
All Residential Buildings 116.9 67.2 4962.9 794 999.8

Source: https://www.nrel.gov/docs/fy18osti/70901.pdf

Parity in Adoption

In addressing the question regarding the feasibility of achieving parity in solar access across income groups, the authors framed their analysis around recent Department of Energy (DOE) solar cost goals.  NREL recently found that achieving these cost goals could lead to solar providing 33% of total electrical generation in the US by 2050. Driven by this goal, the authors investigated if it is possible for rooftop solar to offset 33% of household electricity consumption—for both LMI and non-LMI households.

Although the authors note that the data does not allow analysis at the household level, they were able to show that offsetting 33% of LMI electricity consumption at the national level is possible while only considering the technical potential of LMI single-family owner-occupied residences. However, producing this fraction of LMI electricity consumption would require a DPV system to be present on the rooftop of every LMI single-family, owner-occupied home, which is unlikely to happen. Since the majority of LMI rooftop area exists on renter-occupied and multi-family residences, considering the generation produced by this additional building stock could bring the total offset to 92% of low-income and 80% of moderate income household electricity consumption, which are well above the DOE’s projection. Meeting this goal would require just about two-fifths of LMI rooftop space to be covered with PV systems, which is a more realistic target to meet by 2050. This would still require significant DPV penetration into renter-occupied and multi-family residences, which have seen low levels of adoption to date.

On a more granular level, the authors also add that this technical potential can only be met at a national level, and that adoption could vary between counties. Although some counties will not be able to offset 33% of LMI electricity consumption, nearly 99% of U.S. counties have enough in-county rooftop potential to offset this goal.

To round out their analysis, the authors considered whether installing solar on other types of buildings that LMI communities interact with could help counties meet this DOE projection.

Buildings that Serve LMI Populations

To serve counties where 33% of electricity consumption cannot be offset by LMI rooftops and to attempt to offset 100% of total LMI electricity consumption nation-wide, the authors also evaluated the ability for public buildings like schools and churches to oversize systems and export energy to LMI communities. For example, a school could build a solar array on its roof that produces more than 100% of the building’s consumption and export the excess energy to LMI residents within the same county. Exporting arrangements could provide between 1.3%-8.7% of LMI electricity consumption depending on the area in question.

In exploring approaches to reduce LMI energy burden and increase parity in access to solar energy, several states have instituted LMI carve-outs as part of their community solar policies. These policies typically mandate that a certain percentage of subscribers come from an LMI income group, but do not explicitly make developers site projects on buildings that serve LMI communities. Other programs, such as Washington, DC’s Affordable Solar Program have designed and installed solar panels on LMI rooftops. Even though some classes of public buildings could oversize PV systems to export energy to LMI communities, this largely remains an untapped and untested resource.

Summary

As the cost of solar technologies drops, barriers to LMI solar deployment may center on PV system ownership arrangements. The NREL report demonstrates that through new ownership mechanisms, a large majority of LMI electricity consumption could be offset by distributed solar PV systems sited on LMI residential buildings. Achieving this full potential will require innovative deployment models that enable easier access to rooftop PV or community solar systems for LMI renters and occupants of multi-family buildings.

The full report is available at https://www.nrel.gov/docs/fy18osti/70901.pdf.

All data used in this report is available online on the NREL website: https://www.nrel.gov/docs/fy17osti/68105.pdf

NREL has also developed a Solar For All data explorer that visualizes U.S. rooftop solar technical potential and low-income energy use in the U.S. residential sector. This app can be accessed here

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