Install Inequality: Nearly half of U.S. residential rooftop solar potential is currently out of reach

poor apartment buidlings

One of the largest barriers to solar adoption on a wide scale is the wealth gap, and it will require more problem-solving than a mandate to overcome it. A new report released by the National Renewable Energy Laboratory (NREL) shows that nearly half (42 percent) of all the United States’ residential rooftop solar technical potential (see pg. 15 for definition) is on the dwellings of low-to-moderate income (LMI) households, representing 330 GW of potential solar capacity — a number the researchers admitted was much higher than they expected at the outset.

“Understanding the potential size of the LMI market in detail offers new insights and opportunities to serve these communities,” said David Mooney, executive director, Institutional Planning, Integration and Development for NREL. “The potential electric bill savings from the adoption of rooftop solar would have a greater material impact on low-income households compared to their high-income counterparts.”

Although residential solar adoption has increased over the past decade, adoption among LMI households (defined as 80 percent or less of the Area Median Income) and affordable housing providers continues to lag.

The obvious issue here is the lack of capital, cash or credit for such an investment among LMI customers, but the NREL report also shows how solar financing strategies and the long-time inability to penetrate the multifamily sector specifically leaves behind the LMI segment.

Segment spotlight

Across the entire U.S., all income levels mushed together, the rooftop potential of residential single-family is much higher than multifamily — 68 percent versus 32 percent — but the high-income category is doing the heavy lifting to get that outcome. Splitting this chunk another way, into owner-occupied and renter-occupied, reveals where the LMI segment diverges from higher income categories. After doing this, the largest modality of potential is single-family owner-occupied (SFOO) at 177 TWh, but is closely followed by multifamily renter-occupied, with 140 TWh of potential.

Said another way, although deployment of rooftop has been concentrated on SFOO, about 60 percent of potential is in the other three combinations. This means over half of LMI technical potential for solar is in underrepresented housing combinations, like single-family renter-occupied and multifamily buildings, which means the barriers of solar deployment in these categories is really an additional a barrier for an LMI individual’s access to solar.

The study shows the quantity of residential technical potential is highly concentrated in urban and densely populated areas with more building stock, which makes sense intuitively. But many of these areas with high levels of potential already have significant levels of residential deployment, like California, Maryland, Massachusetts and New Jersey. Several states cited to have high potential with low levels of deployment were Illinois, Ohio, Florida, Pennsylvania and Texas.

RELATED: Solar for All: How to incentivize community solar projects to benefit low-, middle-income customers

At a high level, patterns of LMI potential mirror overall income trends. LMI solar potential percentages are greatest in the lower income communities and higher in rural counties. Spatial trends in the potential for solar to offset LMI consumption most strongly reflected regional variation in per-capita electricity consumed, primarily due to which fuels are used for building heating and cooling loads.

Impact on the future

The Solar Energy Technology Office of the U.S. Department of Energy updated the cost targets of 5 cents per KWh for residential solar by 2030. Using these costs and making forecasts, NREL estimated that achieving them would result in 970 GW of PV capacity 2050, or 33 percent of the generation mix. But can we hit that target leaving the LMI solar rooftop segment in the dust?

Using this data set, NREL examined the feasibility of rooftop offsetting that much in each county in the United States given the technical potential.

Offsetting 33 percent of LMI household electrical consumption (“offset target”) with rooftop solar is technically feasible on a national scale when only considering households in SFOO buildings, although to do so requires buildout on essentially all SFOO buildings — an impractical and unforgiving market challenge. In contrast, on a technical basis, there is more than sufficient roof space to meet the 33 percent offset target when including single-family rental-occupied (SFRO), multifamily owner-occupied (MFOO), and multifamily renter-occupied (MFRO) buildings.


Using only the popular SFOO segment, 60 percent of counties would have potential to meet 33 percent of LMI electricity consumption. Said in reverse to belabor the point: 40 percent of U.S. counties have insufficient rooftop potential to offset 33 percent of LMI electric consumption in just single-family, owner-occupied. The current path to 2050 will not achieve the target generational mix. But including rental and multifamily there is “more than sufficient rooftop space to meet the 33 percent target.” NREL believes 99 percent of counties would meet the 33 percent threshold in just residential rooftop capacity.

Reaching this potential requires deployment models other than those commonly found today. Such models would need to ensure the rental owner had incentive to install solar on their own buildings, like bundling utility expenses with rent payment as a means of passing the costs and savings along to tenants. These models would also need to address diverging requirements and energy burdens of owners and tenants in multifamily. Here’s a great example of one new concept in our Attack the Tariff Series.

It takes a village

The NREL study shows that most LMI electricity consumption (especially with the LMI segment requiring a lower threshold to offset onsite) can be met with rooftop solar, but again 100 percent deployment is unlikely. To get to these high levels of penetration would require deployment on non-traditional building types.

NREL posits one way to increase LMI access to solar is through the vast network of nonprofits that connect to this segment. What if PV systems on government buildings, public housing, schools, shelters and places of worship were intentionally oversized to benefit their LMI communities with the excess generation? The NREL team estimated this opportunity for three cities — Chicago, San Bernardino-Riverside and Washington, D.C. — based on building size, average electric consumption and solar technical potential for outsizing each of those buildings segments. They found enough gross generation potential on those selected building types to meet between 10 to 30 percent of LMI consumption, but only about 1.5 to 9 percent after accounting for the onsite consumption.

Schools have the greatest opportunity to export to the community because of their typically large flat roofs and lower levels of electrical consumption in the summer when irradiance is highest. Places of worship came next because of low levels of consumption year round and moderately favorable roofs. Public housing sites and homeless shelters likely have insufficient rooftop areas to offset 100 percent on site consumption.

National nonprofits GRID Alternatives and Vote Solar updated their Low-Income Solar Policy Guide which explains some proven strategies for expanding solar access being used in states and cities across the country. In multifamily, for example, successful strategies include:

  • Net metering or other incentives to ensure full value of solar
  • Financial incentives to reduce upfront costs, overcome split incentives scenarios and ensure benefits reach tenants
  • Measures to reduce barriers to financing
  • Technical assistance to affordable housing providers, participating contractors and service providers
  • Pairing solar with energy efficiency programs
  • Facilitating waivers from regulatory utility and rent allowance requirements to maximize tenant benefit. (Under a utility allowance formula, a resident’s rent plus utilities equate to a certain percentage of the resident’s income. When a resident’s utility bills decrease, as can happen with solar, the rent portion will automatically increase under the formula)
  • Integrating job training and employment opportunities in the solar energy and energy efficiency sectors of the economy

California and Washington D.C. are the only examples of active programs in place specifically targeted to deploying solar for multifamily affordable housing. California has an incentive program dedicated to affordable housing multifamily, with requirement that half of energy generated on site be used to serve tenants loads. Other states have included incentives for multifamily solar adoption in their broader solar programs. In Colorado, the Denver Housing Authority’s 2-MW LMI solar garden model has shown a scalable model through utility partnerships for offsite generation. In Massachusetts, the SREC II program has awarded a higher price for solar renewable energy credits that are generated by projects that are considered community shared solar projects or that serve affordable housing. When the SREC II program ends, it will be replaced by the new Solar Massachusetts Renewable Target (SMART) program that will award a higher incentive for solar projects that serve affordable housing.

Final thought

It is a big opportunity, though there are clear market and economic barriers. Ignoring this segment and these potential barriers could significantly limit the long-term size of the rooftop solar market.

“Solar can have tremendous benefits for low-income communities in addition to diversifying and de-carbonizing our national energy mix,” said Tim Sears, chief operating officer for GRID Alternatives. “We hope this research will give more states the data they need to develop effective low-income solar programs and build a more equitable clean energy economy.”

The report is accompanied by a web application ( that enables users to assess solar technical potential for their communities. This tool makes it possible to visualize the amount of low-income solar potential in a specific neighborhood, for example, while also enabling identification of neighborhoods with both high solar potential and high electricity costs where rooftop solar could provide cost-effective electricity generation. Check it out and see if it sparks any new ideas. The potential is there, it just needs to be tapped.


Using LIDAR data from Homeland Security to examine 23 percent of U.S. building stock, the researchers inferred the solar potential of building footprints and unshaded roof area, azimuth, tilt and roof plane. Age cannot be detected so was not considered. This was then matched with socio-economic demographic data from the Census and building stock data to understand total usable rooftop area for LMI households. A statistical model was then created to make estimates of areas not covered by the available LIDAR data (stuff like household counts, number of suitable buildings, etc.) They then dove into three representative regions to infer more in-depth information.

— Solar Builder magazine

Solar + Sharing: Connect groups of homeowners, renters via one solar + storage network

lithium-ion batteries

The EnSync Home Energy System includes high-efficiency “LFP” lithium-ion batteries, a Matrix Energy Management system with 9-kWac output capacity, modular energy storage capacity of 9-kWh increments, modular DC-DC converters and the DER Flex Internet of Energy control platform.

Brad Hansen, president and CEO of EnSync Energy Systems, believes solar + storage for the home is still “in the dark ages.” EnSync Energy Systems has built a reputation for deploying high-value distributed energy resources (DERs) in the C&I segment. So why is Hansen discussing residential solar + storage? Well, EnSync has just launched a Home Energy System that will not only address some of the antiquated architecture of current home storage systems but also invent an entirely new project design concept.

At the basic level, the EnSync Home Energy System combines solar, energy storage, power electronics and Internet of Energy control into one platform. It has the advantage of leveraging technology and lessons learned from EnSync’s C&I business to achieve an outcome like lessening thermal stability concerns of repackaged lithium-ion batteries that are often used in a home energy storage applications. Instead, EnSync pairs a residential-scale version of its modular Matrix Energy Management system with thermally resilient lithium-ion batteries and its DER Flex Internet of Energy solution that are all designed to work together.

“Most [current systems] are significantly underpowered and cannot support the entire home if the grid electricity is out, or they have issues disconnecting and reconnecting to the grid during an outage,” Hansen says. “If the home is off-grid, many cannot reliably perform if high inrush currents are created by the start-up of appliances like refrigerators or air conditioners.”

Peer-to-peer exchange

This is the part that could change the sector. The EnSync Home Energy System introduces True Peer-to-Peer energy exchange technology. The goal here is to enable individual residential units in a property to be linked into a network behind the utility meter to provide highly efficient, direct energy exchange between units. Suddenly property developers, property managers and homeowners’ associations can provide seamless and economical transfer of excess energy from any given residence to any other residence in the network with excess demand via EnSync’s DC-Link.


“The future of the electricity market will be individual homes and building owners operating in a ‘spot market’ for the buying and selling of electricity across a network,” Hansen says. “At EnSync, our mission for the company is simple: the democratization of energy through innovative and economic energy systems. Homeowners and property owners that install our products today do so with the confidence that as the market for energy continues to be radically changed, they are not only prepared for it, but can capitalize on it and profit from it.”

A single residence, multifamily building or entire neighborhood could reduce consumption on the grid and possibly open up a new revenue stream. The sharing of electricity between interconnected residences on a True Peer-to-Peer energy exchange network prioritizes the use of solar generated or stored electricity ahead of that from the utility grid for any residence in the network. The network can also be configured as “non-export,” meaning no excess generation for any unit goes back to the utility grid. This capability is becoming more critical as several states and jurisdictions prohibit or economically penalize energy export.

Additionally, many utilities are in the early stages of implementing time-of-day electricity rates and are already levying punitive demand charges on customers. The evolving rate structures and impact of resident vacancy rates, vacation schedules and time-of-day load profiles frequently make deploying solar generation uneconomical for large portions of property development. Virtual net-metering and virtual peer-to-peer programs are fraught with excessive complexity and administrative overhead.

EnSync will initially target the multi-residential property market for its solution, and then broaden its market presence. At the time of launch, the company had already built a sizable order backlog for the EnSync Home Energy System. The Michaels Development Co. was the first to sign a 20-year PPA to build a solar and energy storage system at the Keahumoa Place affordable housing development, a greenfield project in Hawaii that is expected to complete construction in 2019. Savings from the PPA will finance the construction of a 750-kW PV panel-covered canopy that will simultaneously produce energy and shade the development’s parking lot, as well as a 500-kW hour energy storage system, with individual modules interconnected by the proprietary True Peer-to-Peer DC-Link behind each unit’s utility meter.

“True Peer-to-Peer revolutionizes the economics of solar + storage in residential properties like Keahumoa, by dramatically reducing the negative impacts of vacancy rates, absence during peak generation times, vacation schedules and micro-loading effects within each unit from appliances such as refrigerators and air conditioners,” Hansen says.

— Solar Builder magazine

Photovoltaic facades: How feasible is the technology, and in what applications

BIPV solar facades

There are several flavors of technology today jostling to make the dream of electricity-generating windows a reality. While these are likely too expensive and inefficient to be in your home any time soon, pilot projects for such windows in commercial buildings are gaining momentum, and studies (utilizing annual data of sunlit areas in cities) have claimed potential gigawatt-hours of electricity generation in the windows on a high-rise. The graphic below from the new IDTechEx Research report “Smart Glass and Windows 2018-2028: Electronic Shading and Semi-Transparent PV” gives an overview of some of the conventional and emerging photovoltaic (PV) technologies.

Today, the flavor of this technology with the largest established base of installations is of crystalline silicon (c-Si) manufactured into strings and laminated in glass, which has been sold since 2009. One reason for this is that c-Si is some decades old and is proven to last in performance and aesthetics for north of 25 years: this reassures building owners and designers. The weakness is the non-uniform transparency (lines created by the c-Si strings giving a ‘grating’ effect), and a ceiling of 60 percent visible transparency. Ultimately this results in a disadvantage that so far has prevented the market from truly taking off.

However, there is a strengthening case for use of this glass horizontally placed, like in a skylight, or integrated into the building envelope as a facade. In other words, glass areas that are not meant to be looked through. By our estimates at IDTechEx, the volume of this market is to the order of hundreds of thousands of square meters a year and growing.

An emerging thin-film PV with promise is organic photovoltaics (OPV). The main advantages of OPV are a potential for over 80 percent visible transparency (which is comparable to conventional glazing), a flexibility, which means a curved glass is possible (for, say, an electric vehicle, where there is also a trend for an increasing glass area per car), and it is lightweight. An added benefit is that the high transparency is achieved by being more reliant on other parts of the electromagnetic spectrum – UV and infra-red – which provides the advantage of blocking heat entering a building, allowing you to turn down the power setting of your air-conditioning and save electricity.

solar OPV IDTechEX

OPV is just beginning to be commercialized, and a handful of companies in the space are making progress. One company called Sunew has already installed OPV in a skylight in São Paulo. While this is a major step, it is not highly transparent and at a distance looks similar to what conventional PV looks like stuck on a flat surface. Other companies interviewed by IDTechEx have attracted partnerships with members of the big four glaziers (AGC, Nippon Sheet Glass, Guardian, Saint Gobain), and have plans for market entry as early as next year.

The Achilles heel of OPV is the low efficiency (which is below 5 percent, often below 3 percent), and the lifetime and stability. OPV companies typically quote lifetimes that range between five and 10 years, depending on the application. Our understanding at IDTechEx is that 10 years would be for a static application such as a commercial building or high-rise, and a car, which is subject to more shocks, would sit at the five-year end of this spectrum (or worse). Anything below 25 years of life is a tough sell for the construction industry, which means advancements are likely required before mainstream adoption in this segment.

IDTechEx has recently published a major update to its market report on Smart Glass, which covers electricity-generating glass based on organic photovoltaics (OPV), solar concentrators and more. Check “Smart Glass and Windows 2018-2028: Electronic Shading and Semi-Transparent PV” to learn more.

Luke Gear is a Technology Analyst at IDTechEx.

— Solar Builder magazine

Solar project logistics: Calculating the value of an efficient supply chain

solar inverter supply chain

What is a supply chain? It is the flow of all components that go into a given product, and then the flow of that product from its manufacturing origin to its end destination. In a global economy (yes, there still is one, despite Trump’s best efforts) the path from cradle to application can get wildly complex. Inverters are a great example. Modern inverters have thousands of components, integrated into sub-assemblies and then into the inverter product. Sometimes this is happening countries and oceans away from the final point of installation.

As inverters become more homogenous in their basic functions and reliability, finer elements of performance will define the strengths and weaknesses of inverter suppliers. Each supplier’s own ‘supply chain’ is one of those competitive performance variables that buyers and system designers need to consider.

RELATED: How to maximize large-scale PV site value with string design


An optimized supply chain is valuable to project owners for all of the inherent benefits of efficiency:

  • Costs will likely be lower
  • Lead times will likely be shorter
  • Fewer steps from A to Z means less risk of errors occurring
  • Adjustments will be easier to make on the fly

As it relates to inverter suppliers specifically, developers and EPCs should consider the following.

1. Follow the path of the inverter in reverse, from PV application location, upstream to the site of manufacture.

Does the path make sense? Consider how many stops and warehouses are involved because every stop and transition slathers on another layer of risk and cost.

Beyond the physical transition from place to place, how many changes of ownership are involved in the movement of goods? Is a third-party warehouse used, or a third-party distributor required? Every hand-off means transition of ownership (title and/or process) and usually means added cost and markup by each party. The end owner of the inverter pays for all this, so make sure whether or not you are paying for added risk or added value.


2. Product line strategy – the higher quality inverter could come at a better price point depending on the product line and the supply chain.

How many options, variables and models are you dealing with? Sometimes too many variables means an increased risk of management mistakes and costs, as well as more overhead to manage more parts. Contrast that with a line of a few feature-rich, flexible product models that work for a variety of applications. This can be easier to manage and is helpful for designers.

Now, will feature-rich options add to costs? Not necessarily because of the economies of scale gained from producing fewer models. A localized inventory is more feasible with fewer products to focus on, leading to shorter lead times and ready-to-go stock and reduced inventory investment. Applications engineering, service and life-cycle support is also easier to manage with fewer products for both buyer and seller.

3. Get a full picture of all variables.

Weigh the pros and cons of a supplier manufacturer versus a third-party contract manufacturer. Consider the proximity of the fulfillment hub to the user and the carriers used (is it FedEx or some random company?). Just remember that a fulfillment hub or “Made in the USA” sticker doesn’t give the full picture. The global center of power electronic component production is Asia. So an inverter fully pieced together in Asia that ships to a U.S. fulfillment hub may actually be the most efficient supply chain you could find.

We will also dive into this in MUCH greater detail in this upcoming free webinar. Sign up here.

Utility-Scale String Design

Wed, Jun 20, 2018 2:00 PM EDT

When designing a large site one consideration is String or Central. Both have well defined benefits. Historically, the large utility-scale sites have mainly relied upon central inverters. Now a third option, the Virtual Central, is paving the way for string inverters into this space. In this webinar, we will discuss the benefits and disadvantages to both the distributed and centralized string architectures and how the design choice affects installers, developers and site owners. Sign up here.

— Solar Builder magazine

Power factor boost: Make sure to maximize revenue in design, data monitoring

inverer power factor

Remember the Seinfeld episode where the rental car company took Jerry’s reservation, but still didn’t have a car for him“You know how to take the reservation, you just don’t know how to hold the reservation. And that’s really the most important part: the holding. Anybody can just take them.”

That’s a way to think about monitoring performance of a solar site. (Just go with me here) Anyone can just monitor data, but the key is knowing what to do with it – being proactive versus reactive. Here’s two considerations for being proactive involving inverter selection.

Factoring for power factor

Utility-scale sites often come with reactive power requirements, which usually means reducing the real power produced to provide reactive power support. Because of this, be sure to check the power factor or Max AC Output Power of inverters you spec.

“When you reduce active power, you’re not getting paid at what you designed the system for,” says Sarah Ozga, product manager at CPS America. “We want customers to get paid for the nameplate rating of the inverter and not get dinged for reactive power requirements.”

CPS inverters, for example, come with kVA overhead and will supply 100 percent active power while accommodating reactive power requirements. For a 100-kW inverter, this is listed as 100 kW / 111 kVA at PF greater than 0.9 and 125 kW / 132 kVA at PF greater than 0.95 for the 125-kW inverter. Let’s play out some scenarios.


“If we had a 100 kW/111 kVA or our 125 kW/132 kVA rated inverter and the utility company said we need to run at .95 power factor (PF) we could do this without sacrificing real power (kW),” Ozga says. “For example, if we have enough PV power coming in from the array to produce at max capability on the 125kW/132kVA inverter we would be producing 125kW active power and the apparent power would be 132kVA.”

But what if this overhead wasn’t built into the inverters? If the inverter’s apparent power was capped at 125 kVA, at 0.95 power factor (PF), it would be producing 118.75 kW active power. This is about 7 kW less than it could be producing if it had overhead capacity.

“That’s not real significant for a single inverter but these inverters are generally installed in a multi-MW system,” Ozga notes. “So for a 10 MW site that would be 570 kW, which is a significant loss of power.”

Maximize revenue

Over the life of a system, discovering issues early and fixing them quickly can make a huge difference in site performance and, what everyone is here for, revenue. If an inverter goes down or performance is low, make sure the manufacturer is able to remotely troubleshoot, push updates or make setting changes to the inverters without needing to visit the site, like CPS Ameri-ca can within its Flex Gateway. This gets the inverters back up and performing as it should fast-er and cheaper.

“Whether a company is managing its own data or is using a third party monitoring package it is important for the data to be actively monitored,” Ozga says. “That doesn’t mean someone needs to sit in front a computer watching production graphs all day. Set up automated emails/SMS for alarms or warnings for each site that is managed. These warnings could alert you when an inverter is not performing as expected.”

We will also dive into this in MUCH greater detail in this upcoming free webinar. Sign up here.

Utility-Scale String Design

Wed, Jun 20, 2018 2:00 PM EDT

When designing a large site one consideration is String or Central. Both have well defined benefits. Historically, the large utility-scale sites have mainly relied upon central inverters. Now a third option, the Virtual Central, is paving the way for string inverters into this space. In this webinar, we will discuss the benefits and disadvantages to both the distributed and centralized string architectures and how the design choice affects installers, developers and site owners. Sign up here.

— Solar Builder magazine