Blog: American Solar Energy Society

What an incredible weekend! On October 3–5, we celebrated the 30th Anniversary National Solar Tour, and it was one for the books.

This year’s Tour brought together over 800 solar and sustainable sites across the country, from solar-powered homes and schools to large-scale community and agrivoltaic projects, each one inspiring visitors to take steps toward clean energy adoption in their own communities.

Thanks to all the hosts, volunteers, and visitors who made this milestone year so impactful! Your enthusiasm and commitment to renewable energy are what make the Tour the nation’s largest grassroots solar event.

We’ve received inspiring stories and photos from Tour participants nationwide, showing how local action continues to drive the clean energy transition. Together, we’re proving that solar works everywhere, for everyone. View the solar tour map at map.nationalsolartour.org.

2025 National Solar Tour Highlights

How to Attend In-Person Tours: Although the showcase weekend is over, there are still in-person tours taking place all throughout the rest of the year! Check out the National Solar Tour map or digital app to RSVP to in-person tours near you to view the exact address. Optionally, some hosts have made their street address, email address, or phone number viewable in the tour description as well.

How to Attend Virtual Tours: View all the virtual tours featured on this year’s National Solar Tour, simply head to our National Solar Tour map and start selecting the pins you’re interested in, or use the filter feature on the top left hand corner of the map. These virtual tours contain various photo and video tours featuring a wide range of sustainable features.

© Ben Zook

Ben Zook’s Solar Home: Ben Zook, owner and NABCEP Certified Master Electrician at Belmont Solar, hosted a passive solar open house on October 4, featuring a 33 kW solar PV system with 40 kWh of onsite storage, enabling off-grid operation. Upgrades to insulation, windows, and skylights improved comfort while maintaining the home’s original design. Powered entirely by renewable energy with no fossil fuels or gas connections, the home included EV charging, a ground-source heat pump, and all-electric appliances such as an induction cooktop and heat pump systems. With exceptional indoor air quality and abundant natural light, the home operated beyond net-zero.

© Brett Little

Little Residence GreenStar: Join Brett Little, Education Manager at the GreenHome Institute, who wants to invite you over to his home (virtually) as part of the American Solar Energy Society National Solar Tour! Discover how he transformed a 2002-built residence into an all-electric, solar-powered home, achieving a DOE Home Energy Score of 10, PEARL Platinum Certification, and the ultimate holistic approach using the GreenStar Homes Certification. This tour will guide you through the process of transforming a traditional home into a sustainable showcase, covering every step from initial green inspections to upgrading heating, cooling, ventilation, appliances, plumbing, and more—all while maximizing comfort, safety, and utility cost savings.

Utility-Scale Oxbow Solar Project: The Oxbow Solar project is a utility-scale solar farm generating 345 MW-dc / 300 MW-ac of clean, locally produced electricity. Its construction created 400 jobs, the majority filled by local workers, and represents a $394 million private investment in Louisiana’s energy infrastructure—strengthening the state’s energy security. Each year, the project reduces approximately 458,000 metric tons of CO₂ emissions, equivalent to removing about 100,000 fuel-burning cars from the road. Over the next 35 years, it is projected to provide $30 million in revenue to Pointe Coupee Parish government agencies.

© Exact Solar

Exact Solar Local Tour: At Exact Solar’s Local Tour, visitors can explore more than twenty virtual tours showcasing solar installations from their own customers. Each tour highlights unique solar projects from residential rooftops to local businesses, demonstrating the real-world impact of clean energy adoption. These videos showcase firsthand stories from homeowners and business owners about their experiences going solar, including the motivations behind their decisions, the benefits they’ve seen in energy savings and sustainability, and how solar has helped strengthen their communities.

About the Author
In the 1980s, Gale Marsland designed passive solar homes and partnered with a PV installer in the mid-90s doing mostly off-grid. As time went on, she got more involved in the solar community and was a media spokesperson for ASES Buildings Division and on the Editorial Advisory Board for Solar Today magazine. In 2001, she drifted into planning, designing, and building sustainable communities.

For decades, the conversation about renewable energy has centered on technology: panels, batteries, and grids. But the real challenge today isn’t how to capture sunlight; it’s how to finance inclusion. Across the United States, millions of families and small communities still can’t access solar power, even as costs fall and incentives expand.

The solution lies not in inventing new equipment but in reimagining how capital flows. From my experience in energy finance, I’ve seen that access to clean energy follows the same logic as access to credit: it rewards those who already have assets and excludes those who don’t.

A more balanced approach – what could be called blended finance creates shared ownership by aligning government support, private investment, and community participation. It’s not just a funding mechanism; it’s a philosophy of collaboration.

The Limits of Traditional Solar Finance

Traditional project financing favors scale, predictability, and low risk. Those parameters are ideal for utilities and corporations but leave behind rural cooperatives, low-income neighborhoods, and tribal nations.

The barriers are structural: limited collateral, lower consumption levels, and fragmented demand. Yet these same communities stand to gain the most from solar energy with lower bills, local job creation, and energy resilience.

Blended Finance: A Framework for Shared Benefit

Blended finance works by combining three sources of capital that, together, create stability and trust in a project:

• Public capital provides seed funding, often in the form of grants or guarantees, reducing risk for others.
• Private investment supplies most of the funds, motivated by steady returns and clear cash flows.
• Community participation through cooperatives, bonds, or local subscriptions ensures affordability and ownership.

When these forces align, the outcome is a self-reinforcing model: government funds attract private capital, which in turn multiplies community participation. It’s a virtuous cycle that transforms solar projects from external interventions into locally sustained enterprises.

From Concept to Impact

Imagine a solar project serving hundreds of households. Instead of asking who will pay first, the model asks: how can each actor benefit proportionally? The government sets the foundation, private investors bring efficiency and discipline, and communities provide purpose and stability. Together, they form an ecosystem of shared power. The results can be transformative.

Households gain predictable energy costs. Investors secure long-term revenue streams with low volatility. Local organizations from schools to cooperatives gain recurring income through participation fees or ownership shares.

Every dollar invested in generates electricity and increases social and economic value.According to the National Renewable Energy Laboratory (NREL), distributed solar can generate up to 15 local jobs per million dollars invested. Add community participation and those jobs turn into careers sustained by pride, not subsidy.

Lessons from the Field

In several pilot experiences I’ve observed, community-driven financing has changed the tone of renewable projects entirely.

Instead of external developers promising benefits, residents become co-owners of progress. When communities invest modestly, they protect the project, attract talent, and spark a sense of belonging.

The Department of Energy’s Community Power Accelerator shows that partnerships between public agencies and local cooperatives can reduce costs by 15–25% compared to purely private development. Financial innovation, not just engineering, is what unlocks that potential.

Data-Driven Design and Replication

Equitable solar finance needs data and transparency. Projects should start with a financial model that measures social impact and ROI. Tools like Power BI dashboards and open-source models help stakeholders preview project outcomes before installation.

A practical example is creating a revolving fund that automatically reinvests part of community savings into future installations. This mechanism ensures scalability without dependence on continuous government subsidies.

A Call for Collaborative Policy

To scale inclusive solar financing, policymakers must think beyond tax credits. The key is to blend grants with soft loans, standardize community participation models, and simplify permitting for shared ownership.

Programs like the Rural Energy for America Program and state-level clean energy banks are already testing these ideas but coordination remains essential. Financial innovation should be treated as a clean-energy technology subject to research, testing, and replication. By fostering open dialogue between financiers, engineers, and communities, we can ensure that no one is left behind in the solar transition.

The Power We Share

The energy transition will not be won solely by megawatts or tax credits; it will be won by inclusion. Shared power means more than electricity generation; it means shared responsibility and shared prosperity. Blended finance is not charity, it’s strategy. It creates projects that make financial sense while building social capital. The future of solar finance lies in collaboration.

By aligning incentives, empowering communities, and trusting in the multiplier effect of shared ownership, we can turn sunlight into opportunity for all.

About the Author
Christian Custode is a financial strategist and ASES member specializing in renewable energy investment models. He serves as Financial Planning & Portfolio Economist at Ecopetrol USA, Inc and leads Business Planning Consulting LLC, focused on sustainable finance for community solar and tribal energy projects.

Historically, solar panels have not always been feasible in urban environments due to spacing issues. However, new solar technology has recently emerged that can provide renewable energy even in limited spaces.

Balcony solar panels are compact (300–800 watt) plug-in systems consisting of a few PV panels that can be coupled with an inverter and/or battery storage. The systems are designed to be modular and can be fixed upon balcony railings, walls, or stands. As such, they are most frequently installed on multifamily apartment and condominium buildings.

There are a variety of benefits to installing balcony solar panels. Like traditional rooftop solar, they reduce energy bills and reliance on the power grid. For example, one owner interviewed by a German news outlet said he can produce enough solar for half of his family’s daily needs. When compared to rooftop solar, balcony solar does not disrupt existing structures or require access to a roof, solving two major issues that come with traditional solar panels.

Additionally, balcony solar is substantially cheaper than rooftop solar. Differing models range in price from around $400–$2,000, depending on the size of the system. In contrast, rooftop solar installations can cost tens of thousands of dollars and will now be even more expensive with the upcoming termination of federal tax credits.

Nevertheless, there is a noteworthy downside to balcony solar—the panels’ small size means they produce significantly less energy than traditional panels. Moreover, some balconies may have less exposure to sunlight than a rooftop would have, due to shading from higher apartment units or nearby buildings and/or being north-facing. This results in more modest utility bill savings, so balcony solar may not be worth the investment for some apartment owners.

While balcony solar panels have not taken off yet in the US, they have become widely popular in parts of Europe, particularly Germany. Germany currently has around 550,000 balcony solar systems installed, and added 200 MW in the first half of 2024 alone. This is partially because apartment dwellers comprise more than half the population of Germany, and partially because the technology helps combat increased energy prices following the Russian invasion of Ukraine. Many German cities offer subsidies for balcony solar, as well, making the systems even more affordable for consumers.

Conversely, the US has very few balcony solar systems installed. This is largely due to poor regulations that have not kept pace with emerging solar technologies. Electrical interconnection agreement regulations govern how electricity-generating resources, such as solar panels, connect to the power grid. In nearly every state, these regulations include balcony solar panels, forcing balcony solar owners to adhere to strict requirements designed for larger solar projects. Not only does this make balcony solar more difficult to install but it is also an unnecessary measure for a home system of such small wattage. Further balcony solar has not been approved to comply with the National Electrical Code or an Underwriters Laboratories standard—key safety certifications that would spark more interest among apartment owners and make them easier to attain.

Improving regulations to appropriately cover balcony solar panels would help the technology gain traction in the U.S., thereby opening new avenues for renewable energy in urban environments. One California startup, Bright Saver, for example, provides a real-world solution for this problem. The company has begun installing balcony solar systems and marketing them as appliances, rather than construction projects, to bypass strict municipal permitting requirements.

To avoid state interconnection requirements, Bright Saver uses complex energy monitors that prevent any surplus electricity from flowing back into the grid. Unfortunately, such measures are expensive and time-consuming, demonstrating the need for improved regulatory standards. By updating regulations to keep pace with technological advancements, the US can and should follow Germany’s lead and reap the benefits of balcony solar systems.

About the Author
Elle is pursuing a degree in philosophy at the University of Texas at Austin, with a minor in Government and a Business Spanish certificate, and plans to build a career in international environmental law. She is a research associate for Texas Solar Energy Society, where she primarily analyzes a variety of legislative and technological topics. In addition, she has interned with a London-based global sustainable consultancy and conducted alternative energy research for the regulatory department of an electrical engineering firm.

We have been living off-grid for over 35 years and have been involved in designing and building sustainable homes and communities for much of that time. We are determined to pass on knowledge and lessons learned. It has been a major life goal to promote the use of solar energy and other sustainable practices. It is gratifying to see all the advances, but there truly were times when we wondered if solar was ever going to grow, as the resistance from the fossil fuel industry is/was so very powerful and influential.

In a Sahuaro Forest

We moved to this property, an old mining claim surrounded by Sahuaro National Park in Arizona, in 1988. The nearest neighbor and power were almost two miles away. We had been designing and building passive solar homes for some time, and this was our chance, finally, to live off the grid. It has been a wonderful experience, and we have learned many things, refined many concepts, and had great fun imagining ‘what ifs…’ Throughout those 35 years, we have witnessed knowledge, techniques, equipment, and systems steadily improve.

Our house was on the cover of Solar Today magazine, Sept/Oct 1996. We have come a long way since that time. It is beneficial to look at how things have changed and how to advance sustainable practices as time goes on.

Gale, Richard, and Mathew Marsland featured on the 1996 Sept/Oct cover of Solar Today magazine.

Why Do Off-Grid

At the time we built our home, there were many reasons to take on the challenge of designing and building an off-grid home. We:

• Believed in climate change and wanted to contribute in every way possible.
• Considered ourselves to be innovators, problem solvers, and liked high-tech modern stuff.
• Wanted to access the incredibly abundant solar resource where we lived.
• Believed in having a small footprint and finding more ways to do that
• Wanted to be independent from monopolies and institutions.
• Knew that life cycle costs could be less, over 30 years x $166/month = $60K.
• Wanted fewer neighbors and to be surrounded by the natural environment.
• Still hold these values as paramount.

Electricity Supply was the Initial Main Driver

Due to high costs and limits of technology in 1988, of utmost importance to us was to focus on reducing electricity needs as much as possible. Although strategies vary in different climates, some aspects remain the same and have similar solutions. We chose to operate several systems with propane: backup hot water heating, hot tub, dryer, and
space heating.

Energy Efficient Components

Building efficiency into the design of the home, we used the best available insulation for walls and roof, and high-quality windows and doors. We originally used the curliecue fluorescent bulbs, but now we have replaced them with LED bulbs, which use only 10% of the original incandescent bulbs’ wattage. When buying appliances, we chose the most energy-efficient available. The Energy Star website was very helpful. Originally, we used a propane refrigerator, then a super-efficient DC refrigerator that became available later. Now we use the readily available high-energy efficiency refrigerator
and appliances.

Passive Solar for Heating

In this sunny climate, we are able to do 90% of our heating with passive solar. We have a wood fireplace and a small propane heater for cloudy days. We can also use the mini-splits for backup heating. Passive solar components have not changed much and work to a degree in many climates.

The living room is the thermal mass and has south-facing windows. © Gale Marsland

Our house has large, true south-facing windows. They allow the sun in the entire width of the house in the winter and barely come in even a foot in the summer due to the sun’s high angle in the sky. Exterior shutters allow us to control the amount of sun coming in, which is particularly helpful in the spring and fall. They also limit heat loss at night in cool weather, with an added bonus of privacy and security enhancement.

The Marslands’ repurposed the cool tower into a wonderful observation deck. © Gale Marsland

My husband comes from a family of masons, so it was natural to use high mass concepts. The exterior walls and most interior walls are block, and the floors on the first floor are brick. All of this mass stores coolth/warmth and keeps temperatures from fluctuating quickly. The exterior walls are 20” thick with a split-face block on the outside, which blends in with the natural environment. There are 2 inches of isocyanurate foam plus a 2 inch air gap in between, completed by a solid grouted concrete block wall on the interior. We got the mass, the insulation, and a no-care exterior. About a third of the walls are buried underground 3-4 feet, which further provides insulation and contact with even more mass.

Tackling the Biggest Energy User – Cooling

Summer cooling was our biggest challenge. Because of low humidity, we were able to use evaporative cooling and didn’t require air conditioning. Bill Cunningham at the Environmental Research Lab in Tucson helped us address cooling while maximizing solar energy use. Part of our solution was a cooling tower originally developed a thousand years ago in the Middle East, known as a malquaf.

Bill designed a 35’ tall, 10’x10’ tower, with thick evaporative cooler pads on all four sides at the top. Water dribbled from the top down into a gutter and then recirculated. It only used about 50 watts to pump the water! The water coming through the tower cooled the air and, being heavier, dropped passively into the house. Cupolas on the roof vented warmer air out. This resulted in incredibly low power needed for cooling a 2000 square foot area. However, it used a lot of water, about 3-400 gallons a day. With a dry well, we had to haul water. The cool tower also sometimes leaked and stained the ceiling.

Later, we switched to a high-efficiency solar chill evaporative cooler, also designed by Bill, using about 100 watts and only 100 gallons a day, which was a big improvement. Recently, we installed even more efficient AC mini-splits after adding to our solar array and generating plenty of electricity. The mini-splits use about 1000-1500 watts and ZERO water. Our comfort level is greatly improved, and we can just set a thermostat and walk away.

The 2024 PV being mounted directly on the roof. © Gale Marsland

Future Forward: More Photovoltaic (PV)

Components of Active Solar/PV have advanced significantly and are much more efficient. PV modules produce more energy per foot, and therefore take up less space. And prices are much cheaper now, too! Our first solar panels were $25/watt; the ones we most recently purchased were only $0.40/watt! Inverters are also now more efficient. Our original inverters were modified sine wave, and back then, even a few of our electrical devices didn’t work well with that. Modern sine wave inverters are essentially like grid power now. They also put out a lot more power than the older ones: 1,500 W on the modified sine wave versus 12, on my newest one. The controllers and interfaces are smaller, more efficient, and cost less, though not as much cheaper as the extreme reduction of PV modules. We originally used passive trackers to keep the PV 90 degrees into the sun as it moved through the day to maximize output. Now, due to increased efficiency and decreased cost, our newest panels are stationary on our roof.

Another important advance over time is that there are more local businesses installing and maintaining solar systems. Larger national companies are marketing DIY systems and also offer tech support, including Wi-Fi connections that enable technicians to access your system and advise on installation and maintenance issues. When we upgraded, we took advantage of investment tax credits that, unfortunately, are ending on December 31, 2025. Some rebates and incentives will remain in some states and municipalities to help reduce costs, but you have to be in the right state.

Current Challenges

Climate change and extreme weather events are making the grid more vulnerable. Redundancy, duplication, diverse sources, and focus on DISTRIBUTED generation are critical. Our house has two parallel PV systems, and each is backed up by a generator. We also have a transfer switch so that if one system has problems, the other can provide power for it.

A 2017 upgrade included adding an automatic battery watering system. © Gale Marsland

Currently, the vast majority of homes with PV have no storage, back-up, or ability to use the power they are generating when the grid goes down! Most solar companies today are installing grid-tied systems with no batteries. Locally and anecdotally, there are very few companies that are interested in or even capable of installing battery backup systems. There is limited access to service, repair, maintain, or upgrade batteries. This is why so few people make this choice. The hope is that this market will grow and evolve so that it will be more like buying an appliance: bring it home, and plug it in. It is of vital importance to grow the infrastructure for installation, service, and maintenance for these full systems.

The solar community (researchers, tech developers, manufacturers, distributors, installers, community leaders, and organizations like ASES) needs to strengthen and build connections to take on some major portions of the grid in diversity and redundancy. We need to grow influence and power to compete with fossil fuel interests. A goal would be to promote distributed generation, where the individual has major benefits and solar growth is not just in the direction of corporations or utility companies.

Off-Grid Regret

We have only one regret about being off the grid. Seasonally, our power usage changes dramatically, as happens in many climates. Our latest system was designed around summer cooling needs. The other six months of the year, our needs are thirty percent less. So, IF my system fed back into the grid, we would be CONTRIBUTING. But the cost of bringing power up here- the last time I checked in 2018 – was over $60,000. It does not make sense to pay that plus a small monthly fee just to feed the grid. As I write this in October 2025 at 11 am, 17K of my 18K PV feed is already shut off, and batteries are at 100%. What a waste….so our best choice would be grid-tied with battery backup, and GET IT ALL: independence, clean energy, and contribution to the grid.

There are many sunny places, like Tucson, where you absolutely should invest in battery backup.

About the Author
In the 1980s, Gale Marsland designed passive solar homes and partnered with a PV installer in the mid-90s doing mostly off-grid. As time went on, she got more involved in the solar community and was a media spokesperson for ASES Buildings Division and on the Editorial Advisory Board for Solar Today magazine. In 2001, she drifted into planning, designing, and building sustainable communities.

For many people who care about solar energy, caring for the environment is just as important. Trees are a big part of what makes our surroundings feel natural and beautiful. Cutting down a tree to install a solar array might feel a bit defeating.

Knowing the optimal location for installing a solar array increases annual generation by several percentage points if shade from nearby structures and trees is minimized, if not eliminated. Knowing how much energy is lost to shade helps homeowners decide whether to keep a tree or structure, trim it, or remove it. In general, it comes down to minimizing losses with nominal, if any, tree removal. In addition, by knowing the cost of leaving a tree in place (lost generation) homeowners or solar developers can make an informed decision regarding a tree’s potential removal.

Site Selection in the Sun

Sometimes, moving the solar array a little can make a big difference. For example, shifting the panels 20 feet to the east and 20 feet to the north might reduce shade from a barn by 85% and from a tree by about 10%. Moving the array further north can reduce shade even more.

It’s also important to consider the cost of lost energy. If a solar array produces about 8,400 kWh per year, a 5.7% loss due to shade could mean about $48 less in savings each year. Over 20 years, that adds up to nearly $1,000. If moving the array can save that money, it might be worth it.

The first step in estimating shade interference is to determine the solar declination angle (angle between the sun’s rays and the Earth’s equatorial plane), solar altitude angle (angle of the sun’s center above the horizontal plane), and azimuth angle (horizontal direction of the Sun) at regular time intervals for every day of the year. The solar altitude angle determines the length of a shadow while the azimuth angle determines the direction of the shadow. Geographic tools with these angles are readily available on the internet. Suncalc.org, for example, has map imagery and a calculator to visualize where shadows will be cast throughout the year. System designers use more powerful tools like Aurora and Helioscope to optimize siting.

The sun azimuth angle for incremental combinations of solar declination angle (day of the year) and solar altitude angle (time of day relative to solar noon) for any given latitude is calculated. The graph shows the maximum tree height (defined as the maximum height a tree can be without casting a shadow on the solar array at any time during the year) at a radius of 100’ from the solar array with assumed maximum generation losses of 0.5%, 1.0% and 1.5% (based on relatively low intensity of sun light in early morning and late afternoon hours); the given latitude is 38° N.

Maximum tree heights are determined by a small range of dates centered around the winter solstice for the azimuth angles ranging from approximately 132° to 228° with 180° representing due south which occurs at Solar Noon. As the azimuth angle moves lower from 132° or higher from 228° the controlling day for each respective azimuth angle moves from the winter solstice to the summer solstice. Tree heights controlled by the winter solstice reflect days where solar altitude angles are not large enough to clear a tree taller than these limits. This shading from trees and buildings when the sun is at low angles doesn’t actually impact total production very much because the sun is at such a great angle to the plane of the solar array that there would be very little energy generation at that time period even if the trees or buildings were absent. Maximum tree heights for azimuth angles not associated with the winter solstice are dictated by the shallow solar altitude angles that occur in the early morning and late afternoon periods of the day.

A simple way to interpret/use this chart is to assume generation losses will not be exceeded if the surrounding tree/structure profile for the entire range of azimuth angles is below its relevant height limit. Areas where trees/structures are less than the maximum would reduce this annual loss estimate. Perhaps it goes without saying but height limits are proportional to the distance from the solar array.

Estimating Shade Loss

The profile of any object can be defined by a series of combinations of azimuth angles and solar altitude angles. The potential for shading begins when the angle between the object and the western end of the array matches the azimuth angle. As the day wears on, the azimuth angle works its way west to east along the array. Shading ends when the angle between the object and the eastern end of the array matches the azimuth angle.

Trees located closer to due southwest and southeast have a much greater impact on potential generation losses. © Steve Moss

Within this range of azimuth angles, shading will only occur if the solar altitude angle is less than the vertical angle from the array to the top of the object (defined here as the minimum solar altitude angle). Although the minimum solar altitude angles within the range of azimuth angles can be easily calculated, the amount of the resulting lost generation is a much trickier value to calculate. This is because both the azimuth angle and solar altitude angle behave as a sine wave throughout the year. In addition, the intensity of sunlight is a function of the solar altitude angle; meaning the potential for generating electric power diminishes as the sun gets closer to the horizon.

Loss Due to Tree Shading

To determine the potential generation loss of a tree, the distance between the tree and the solar array and the azimuth angle are measured at site. The western end of the solar array is used for both measurements. The azimuth angle can be determined with the compass app on a smart phone by standing at the tree and pointing the phone toward the solar array. The tree height is also estimated (this can be done by someone holding a pole of a known length next to the tree and an observer visually estimating the tree height in terms of the number of pole lengths needed to match the tree’s height). There are, of course, more sophisticated methods of making these measurements.

The tree height, distance to tree, and azimuth angle are entered into a spreadsheet along with the dimensions of the solar array. Azimuth angles and distances to the tree are calculated for the various points along the object’s profiles. These distances are used to determine the minimum solar altitude angle for these various points.

Please refer to the plot sketch for a better understanding of a tree and barn location relative to the solar array. For illustration purposes, the tree’s profile is defined as two concentric cylinders; the first has a 6’ diameter and height equal to the estimated height of the tree; the second has a 12’ diameter and a height 5’ less than the height of the tree. These values can be varied and additional cylinders added, if needed.

In most cases, the shadow width will be less than the width of the solar array. Allowances are made for the fact that generation does still occur within the range of applicable azimuth angles.

In reality, the estimated width at the maximum tree height is an educated guess and will probably be less as the width dimension is taken closer to the top of the tree. In addition, no allowance for partial shading is made. It is also assumed the tree shadow is solid for the entire time it interferes with the solar array and does not factor in space between leaves, nor does it take into account the lack of leaves for non-evergreens during the winter months.

Loss Due to Structure Shade

In most cases, the dimensions of structures can be more accurately measured and shade losses more accurately estimated. Loss estimates are generated with the same method as for a tree.

Structure profiles are constant and not subject to seasonal variances; thus, fairly accurate loss estimates can be achieved and should be weighed accordingly when balancing between competing estimated losses associated with trees.

Optimizing Array Location

Site conditions may limit location flexibility, perhaps a range of trees to the immediate north of the proposed array location. There may also be limits based on aesthetic considerations. Perhaps, if the owner had no choice this could be acceptable, but if the owner could significantly reduce or even eliminate the loss by moving the array it should be considered. Although already well-established, the minimum distance between parallel arrays can also be calculated. This distance increases with latitude; variances in the topology of the area the arrays are being installed must also be considered.

Perhaps the most practical knowledge is the minimum distance required to avoid shading altogether. Another possibility may be determining if the array should be split into two (or more) rows if the site has competing shade producing or space limits to the east and west of the array. Alternatively, if there is a ridge of trees to the south of the array but not much to the east and west, a single solar array is likely the best configuration.

Trees and landscaping provide essential environmental benefits, but always consider how much shade they’ll throw as they mature. Deciduous trees (ones that drop leaves) on the east or west can block harsh summer sun while minimizing winter shading, but keep them far enough south from an array or rooftop system to prevent significant generation loss. Proper solar siting is essential for optimizing a solar array’s location and minimizing the impact to the nearby landscape—green is good!

Practical Tips for Homeowners

• Assess the site: Look at where trees and structures are located relative to the panels. Use a compass app on a smartphone to measure the direction from the tree to the panels.
• Estimate tree height: Hold a pole of known length next to the tree and estimate how many pole lengths it takes to match the tree’s height.
• Use tools and charts: There are online tools and charts that can help estimate how much shade a tree or structure will cast and how much energy might be lost.
• Consider alternatives: Sometimes, trimming branches or moving the array can reduce shade without removing trees. If removal is necessary, weigh the environmental benefits of solar energy against the loss of the tree.
• Consider Future Growth: Trees will grow, and new buildings may go up nearby. Factor this in—trim branches or plan for a little extra space.
• Advanced Panel Tech: If minor shade is unavoidable, panels with microinverters or power optimizers limit the “bottleneck” effect from a single shaded cell.

Example of a 50’ Tree

For illustration purposes a 50’ tall tree located 55’ east and 50’ south of the western end of the solar array is assumed. The range of affected azimuth angles is 130° to 164° for the 6’ diameter portion and 128° to 167° for the 12’ diameter portion. Minimum solar altitude angle ranges from 34° to 41° for the 6’ diameter portion and 31° to 38° for the 12’ diameter portion. The 12’ diameter minimum solar altitude angles are lower because the assumed height is 5’ lower than for the 6’ diameter portion.

This tree will provide an estimated 170 hours of shade and reduce annual generation by about 2.6%. General loss estimates for alternate locations will be discussed in the context of minimizing the total of this tree and a nearby barn later in this article. Note: the 170 hours represent about 3.9% of total hours of sunlight; the difference between this and the loss estimate reflects the lower intensity levels associated with some of the shaded area.

Example: A 20’ tall barn with its northern wall located 20’ south with its eastern wall located 20’ west of the western end of the solar array. All four sides are 40’ long. For simplicity, the orientation of this barn is assumed to be true to the N/E axis. With this barn’s assumed dimensions and location, the associated azimuth angle range is 198° to 257° with the minimum solar altitude angle ranging from 13° to 18°.

This barn would provide an estimated 240 hours of shade and reduce annual generation by about 3.1%. As was the case for the tree example, the estimated loss value is much less than the 5.5% of annual hours where this barn is shading the solar array due to the lower intensity of much of the area affected.

The combined annual generation loss of the tree and barn is about 5.7%. The typical capacity of a 25’-wide array is about 6 kw which would be expected to produce about 8400 kwh’s per year. Applying this loss estimate to the total generation yields annual cost estimates of about $48 based on a value of $0.10 per kwh. Assuming the discount rate equals the inflation rate of electricity, the NPV of this annual cost comes to about $960 based on a 20-year life of the array.

If the array can be moved 20’ to the east and 20’ to the north, estimated generating losses of the barn are reduced about 85% to 0.5% annually while losses tied to the tree are reduced more modestly by about 10% to 2.3%. The losses tied to the tree are reduced by the array shift north but this is almost entirely neutralized by the move east.

Starting at this secondary location and moving the array north by another 20’, losses to the barn and the tree are each reduced to about 0.2%. At this point, the benefit of moving further north may need to be compared to any additional cost or detriment that may be incurred.

About the Authors
Steve Moss received a BS in Mechanical Engineering from the University of Missouri – Rolla (now MS&T) in 1976 and retired from Nooter/Eriksen as a Senior VP in 2015 after 39 years of service. Mr. Moss is a past president of the Academy of Mechanical and Aerospace Engineers at MS&T and is currently a trustee at Ranken Technical College, located in
St. Louis, MO.

Carly Rixham is the Executive Director for American Solar Energy Society (ASES). She is the Publisher of Solar Today magazine. Her research is in the intersection of solar energy and landscape architecture. She received her Masters in Ecology and Evolutionary Biology at University of Colorado Boulder. Rixham was a microbiologist at BioVantage Resources, culturing algae for bio-remediation of nutrients in wastewater. She enjoys travel, art, mountain sports, and nature.

A string of reports released in mid-autumn by the Federal Energy Regulatory Commission (FERC) and the U.S. Energy Information Administration (EIA) confirms that solar and wind as well as battery storage continue to dominate growth among competing energy sources as they add ever more generating capacity and increase their share of U.S. electrical generation.

Solar set new electrical generation records in August and the first two-thirds of 2025: EIA’s latest monthly “Electric Power Monthly” report (with data through August 31, 2025),1 once again confirms that solar is the fastest growing among the major sources of U.S. electricity.

In August alone, electrical generation by utility-scale solar (i.e., >1 megawatt (MW)) ballooned by almost one-third (29.5%) compared to August 2024 while “estimated” small-scale (e.g., rooftop) solar PV increased by 10.8%. Combined, they grew by 24.7% and provided nearly one-tenth (9.5%) of the nation’s electrical output during the month, up from 7.6% a year ago.

Moreover, during the first eight months of 2025, utility-scale solar thermal and photovoltaic expanded by 35.7% while that from small-scale systems rose by 11.0% compared to the same period in 2024. The combination of utility-scale and small-scale solar increased by almost a third (28.8%) and was over 8.9% (utility-scale: 6.7%; small-scale: 2.2%) of total U.S. electrical generation for January-August – up from 7.1% a year earlier.

As a consequence, solar-generated electricity year-to-date (YTD) easily surpassed – by over 58% – the output of the nation’s hydropower plants. In August alone, solar-generated electricity more than doubled the output of the nation’s hydropower plants. In fact, in both August and YTD, solar produced more electricity than hydropower, biomass, and geothermal combined.

In addition, for the second consecutive month, utility-scale solar generated more electricity than the nation’s wind farms – by 4% in July and by 15% in August. Including small-scale systems, solar out-produced wind four months in a row and by almost 50% during August.

Solar and wind are almost one-fifth of total U.S. electrical generation – a larger share than that provided by either coal or nuclear power: Wind turbines across the U.S. produced over a tenth (10.2%) of U.S. electricity in the first eight months of 2025 – an increase of 2.6% compared to the same period a year earlier and 80% more than that produced by the nation’s hydropower plants.

During the first eight months of 2025, electrical generation by wind plus utility-scale and small-scale solar provided almost a fifth (19.1%) of the U.S. total, up from 17.2% during the
first two-thirds of 2024.

Further, the combination of wind and solar provided 16.2% more electricity than did coal during the first eight months of this year, and 11.7% more than the nation’s nuclear power plants. In fact, as solar and wind expanded, nuclear-generated electricity dropped by 0.7%.

Electrical output YTD by the mix of all renewables was over 26% of total U.S. generation: The mix of all renewables (i.e., wind and solar plus hydropower, biomass and geothermal) produced 9.0% more electricity in January-August than they did a year ago and provided 26.1% of total U.S. electricity production compared to 24.5% twelve months earlier.

Renewables’ share of electrical generation is now second to only that of natural gas whose electrical output actually dropped by almost 4.1% during the first eight months of 2025.

Solar was two-thirds of new generating capacity in August and 73% year-to-date: In its latest monthly “Energy Infrastructure Update” report (with data through August 31, 2025),2 FERC says 48 “units” of solar totaling 2,702 MW were placed into service in August, accounting for two-thirds (66.4%) of all new generating capacity added during the month. That represents the second-largest monthly capacity increase by solar in 2025 – just behind January when 2,945 MW were added.

The 505 units of utility-scale solar added during the first eight months of 2025 total 19,093 MW and were almost three-quarters (73.4%) of the total new capacity placed into service by
all sources.

Solar has now been the largest source of new generating capacity added each month for two years straight: September 2023 – August 2025. During that period, total utility-scale solar capacity grew from 91.82 gigawatts (GW) to 156.20 GW. No other energy source added anything close to that amount of new capacity. Wind, for example, expanded by 11.16 GW while natural gas’ net increase was just 4.36 GW.

Including wind, renewables were 88.0% of new capacity: Between January and August, new wind accounted for 14.5% of all new capacity added and provided 3,775 MW of capacity additions. Thus, wind and solar each added more new capacity than did natural gas (3,095 MW).

Consequently, for the first eight months of 2025, the combination of solar and wind (plus 4 MW of hydropower and 3 MW of biomass) provided 88.0% of new capacity while natural gas was just 11.9%. The balance of net capacity additions came from oil (20 MW) and waste heat (17 MW).

Solar and wind are almost a quarter of U.S. utility-scale generating capacity; all renewables combined are over a third: FERC’s data reveal that utility-scale solar’s share of total installed capacity (11.6%) is now almost equal to that of wind (11.8%). If recent growth rates continue, utility-scale solar capacity will surpass that of wind before the end of 2025.

Taken together, wind and solar constitute nearly one-fourth (23.4%) of the U.S.’s total available installed utility-scale generating capacity.

Moreover, almost 29% of U.S. solar capacity is in the form of small-scale systems that are not reflected in FERC’s data. Including that additional solar capacity would bring the share provided by solar and wind to more than a quarter of the nation’s total.

With the inclusion of hydropower (7.6%), biomass (1.1%), and geothermal (0.3%), renewables currently claim a 32.4% share of total U.S. utility-scale generating capacity. If small-scale solar capacity is included, renewables are now more than one-third of total U.S. generating capacity.

EIA confirms that solar and battery storage have dominated capacity additions during the past year, coupled with a strong showing by wind: While not identical, EIA’s capacity data generally track those of FERC.

Between September 1, 2024 and August 31, 2025, utility-scale solar capacity grew by 31,707 MW, while an additional 5,718 MW was provided by small-scale solar. Wind also made a strong showing during the past twelve months, adding 4,792 MW.

Further, strong growth was experienced by battery storage which grew by 63.9% during the past year and added 13,378 MW of new capacity. Battery storage actually surpassed pumped hydro storage (PHS) in October 2024 and now accounts for 50% more storage capacity than PHS.

On the other hand, natural gas capacity increased by only 3,338MW and nuclear power added a mere 46 MW. Meanwhile, coal capacity plummeted by 4,185 MW and petroleum-based capacity fell by an additional 659 MW.

Thus, during the past year, renewable energy capacity, including battery storage and small-scale solar, ballooned by 55,420 MW while that of all fossil fuels and nuclear power combined actually declined by 1,486 MW.3

Solar is on track to become the second largest source of U.S. generating capacity: EIA foresees continued strong solar growth, with even more utility-scale solar capacity – 34,326 MW – being added in the 12 months ending in late summer 2026. EIA also notes that planned battery capacity additions during that time total 20,180 MW, while new wind capacity could add 9,650 MW. EIA’s latest “Short-Term Energy Outlook” report3 looks a bit further into the future. It expects utility-scale solar PV + thermal capacity to grow 22% between the end of 2025 and the beginning of 2027, reaching 182.9 GW, while small-scale solar increases almost 11% to 65.1 GW. During that time, wind capacity would expand over 6% to 169.9 GW as battery storage jumps from 45.7 GW to 65.2 GW.

Meanwhile, FERC offers a three-year outlook and says that net “high probability” additions of solar between September 2025 and August 2028 total 89,953 MW – an amount almost four times the forecast net “high probability” additions for wind (23,223 MW), the second fastest growing resource.

FERC also foresees net growth for hydropower (566 MW) and geothermal (92 MW), but a decrease of 126 MW in biomass capacity.

Meanwhile, natural gas capacity would expand by 8,481 MW and nuclear power would add just 335 MW, while coal and oil are projected to contract by 23,564 MW and 1,581 MW, respectively.

Taken together, the net new “high probability” capacity additions by all renewable energy sources over the next three years would total 113,708 MW. On the other hand, the installed capacity of fossil fuels and nuclear power combined would shrink by 16,329 MW.

Should FERC’s three-year forecast materialize, by early-fall 2028, utility-scale solar would account for 17.1% of installed U.S. generating capacity – more than any other source besides natural gas (40.0%). Further, the capacity of the mix of all utility-scale renewable energy sources would exceed 38%. Inclusion of small-scale solar – assuming it retains its 29% share of all solar – could push renewables’ share to over 41% while that of natural gas would drop to about 38%.

In sum, notwithstanding policy challenges created by the Trump Administration and the Republican-controlled Congress, EIA and FERC both foresee the transition to solar, wind, and other renewables as well as battery storage continuing and accelerating while coal, oil, natural gas, and nuclear all contract.

About the Author
Ken Bossong is the director of the SUN DAY Campaign, a non-profit research and educational organization founded in 1992 to support a rapid transition to 100% reliance on sustainable energy technologies as a cost-effective alternative to nuclear power and fossil fuels and as a solution to climate change.