Technology + Data

Utility-Owned Rooftop Solar


Arizona Public Service (APS) is one of the top-five, highest solar-penetrated utilities in the nation, with more than 930 megawatts of solar energy available on its system today. This is enough to power more than 230,000 homes when the sun is at its peak.

As the largest, longest-serving electric utility in the state, with 1.2 million customers, APS has been a consistent fixture in the top quartile of independent energy rankings for customer service, reliability and safety.

Now the Phoenix, Arizona-based company is breaking new ground with its APS Solar Partner Program, the nation’s first, utility-owned residential rooftop solar program dedicated to studying the capability of advanced inverters, operating in real-time, within an electric utility’s system.

APS sees its program as a long, sought-after solution to the utility industry’s effort to modernize its infrastructure; and to keep pace with the rapid expansion and development of advanced technologies, and distributed energy resources (DERs) such as solar, battery storage and home energy-management systems.

Through a unique and exciting research and development program, APS intends to employ multiple “use cases” that will lead to a better understanding of the byproducts stemming from the highly technical, complex interactions occurring every second that DERs are employed on the APS grid.

In fact, APS engineers contend that the program’s success hinges on their ability to map a proverbial electronic communications “genome,” which, once completed, will inform the utility as to how to best manage what APS refers to as the “smart, inverter-advanced meter communications nexus.”

Breaking this code will enable APS to set a proper roadmap to harness the sun’s power, and in doing so, preserve the company’s stalwart record of providing safety and affordability through reliability-oriented operations from an advanced distribution operations center.

The utility industry must modernize its infrastructure, rate structures, and be more agile and flexible in the way it delivers service to its customers.

APS believes that the utility industry must modernize its infrastructure, rate structures, and be more agile and flexible in the way it delivers service to its customers.

The APS Solar Partner Program is a useful and purposeful step in the right direction.

A Legacy of Solar Research

APS is no stranger to research and development in the solar industry. In 1954, APS led an organized effort to launch the first-ever International Solar Energy Exposition held in Phoenix.

In the 1980s, the utility commissioned the Solar Test and Research Center (STAR Center) to serve the research needs for APS, Arizona’s budding solar industry and academia from around the world. They regularly turned to the STAR Center to test solar technology’s latest advancements.

APS went on to build Arizona’s first commercial solar power plant in 1997 in nearby Flagstaff, and in 2004, it was the first in the world to install a grid-tied solar system at the STAR Center, using gallium arsenide, triple-junction photovoltaic cells.

APS officials state that a “STAR Center 2.0” could be on the horizon within the next few years as the company continues its industry-leading research in DERs.

The Story Behind the Story

The APS Solar Partner Program originated from APS’s recognition that it needed to learn more about the integration of DERs on its system.

The company first explored these ideas in 2010 through an APS-led High-Penetration Solar Deployment (HPSD) study in Flagstaff.

This research, funded by the U.S. Department of Energy, was ideally suited to be performed on a high-penetration solar feeder in APS’s newly installed Flagstaff Community Power Project (Flagstaff CPP).

In the Flagstaff CPP, APS owned, operated and received energy from solar panels on eligible residential customer sites.

Five years later, under the APS Solar Partner Program, APS will install, own, operate and receive solar energy exclusively from residential customers; and the utility will pay customers $360 per year for 20 years for the use of their rooftops.

The Solar Partner Program, at a scale of 10 megawatts and 1,500 customers, is emerging as a more advanced and more sophisticated successor to the 125-home Flagstaff CPP.

The risk-free initiative is such a good deal for customers, its first participant, Gregg Dufort, of Phoenix, coined it a “no brainer.”

Behind the Scenes

The HPSD and the Flagstaff CPP can be categorized as gifts that keep on giving. APS has been able to capitalize on valuable data and research from both projects to drive the sophisticated, highly technical planning for the Solar Partner Program.

Consecutive months of robust customer demand for solar in its service territory, and the previous HPSD research on the CPP, ultimately led APS to the decision to pursue the Solar Partner Program.

APS believes its program will enable the utility to capture national attention for creating an actual smart grid that embraces an abundance of solar photovoltaics (PV) in its system.

With more than $3 billion already invested in solar throughout the state, and the recent completion of its eighth and ninth plants under its 170-megawatt AZ Sun Program this year, APS officials are working to enhance a grid platform that will support more DER.

They see the Solar Partner Program’s research and development as the crucial first step.

APS sought input from its customers and the solar industry when they considered an alternative to expanding the company’s solar portfolio exclusively through utility-scale solar plants, as was the case with AZ Sun.

Instead, engineers realized that a community rooftop or distributed generation solar plant could produce a high number of technical learnings about advanced inverter integration, which has never been done before in the history of solar deployment.

The result was the creation of a program that essentially amounts to a solar plant dispersed across 1,500 different systems, with each individual module subject to control by the utility, if necessary, to meet system requirements.


Under the Hood

The Solar Partner Program’s use cases will test various scenarios on the APS system:

  • Equipment deferment
  • Voltage management
  • Stress (high solar penetration and interoperability)
  • Inverter control and functionality
  • Communication
  • Energy storage
  • Model validation
  • Solar forecasting

APS further maintains that embracing the interaction of grid and customer technologies will help the utility be an enabling platform for other DER technologies, such as rooftop solar and battery storage, or both. APS sees this utility distribution system as a more complex and more prominent component of the utility’s future business model.

From the outside looking in, most people take for granted the complex interactions taking place from the rapid increase of DERs being introduced onto a utility’s system.

As a result, APS will also utilize the study to create a blueprint for a secure, reliable and protected communications protocol, which can seamlessly integrate with national critical infrastructure protection requirements.

Likewise, the communications development and innovation APS intends to conduct through these use cases will play a central role in its ability to plug and play with all kinds of different customer technologies that exist or that are coming online today and tomorrow.

Collaborative Research and Development

New questions arose from the HPSD study and the Flagstaff CPP about the need to better understand important operational issues such as:

  • feeder model validation,
  • real-time feasibility of dynamic control from an advanced distribution operations control center, and
  • other advanced inverter interaction (such as voltage regulation and reactive power support).

As a result, APS is collaborating with the Electric Power Research Institute (EPRI) to validate the research and development conducted throughout the program.

EPRI, likewise, has added Solar Partner to its integrated grid research. Solar Partner is a national initiative aimed at optimally using renewable energy in the most beneficial way to ensure that customers continue to receive reliable, affordable, secure and environmentally responsible electricity.

APS intends to use its findings from the study to inform other utilities across the country about how well solar power can be used on demand, and tapped into as required, to help divert or supply extra power to the grid.


Community Engagement

Earlier this year, APS selected eight local Arizona solar contractors as the program’s official installers. It’s an important move that the company believes will support the local economy as the utility seeks to promote DER interaction throughout Arizona.

APS also sees a value in collaborating with key stakeholders, community groups, other utilities and manufacturing representatives to determine how the program will best reflect both the long-term energy needs of APS customers and the future of solar energy in Arizona.

APS formed an advisory panel with representatives from the Residential Consumer Utility Office (the state’s leading consumer advocate), the National Renewable Energy Lab, Arizona State University, Arizona Corporation Commission (ACC) staff and others.

The goal is to ensure that APS maintains a close relationship with the community organizations most aligned with customer needs.

At the strategic level, APS sees the Solar Partner Program and others like it as critical elements that will enable the company to continue its mission of promoting a sustainable energy future for Arizona, a dominant theme that has been a hallmark of the company’s longstanding history.

Given the company’s 60-year track record of leading solar research and initiatives, APS executives are leveraging the company’s deep community ties and its experience in innovation to take the utility to the next level in DER and grid interaction.

APS is open about its efforts to direct the market of the future, to harness the benefits of distributed and renewable resources and to be on the leading edge in the high-penetration solar arena.

Engineers are firm in their convictions that Solar Partner and other projects like it will create learnings that will not only benefit the APS system and its customers in a meaningful way, but will also benefit of the entire installation of solar throughout the United States.


Demonstrating solar leadership

APS is putting its money where its mouth is in terms of leading the integration of DERs onto its grid.

In a separate initiative, APS is planning to launch the Solar Innovation Study (SIS), a pilot effort designed to give customers more control over their energy environments. Characterized as a “rate laboratory,” SIS will test ways to enable customers in 75 single-family homes to control their energy demand through the employment of DERs, such as solar and battery storage, variable-speed HVAC systems, smart thermostats and load controllers in their homes. A better understanding of how these consumer-oriented technologies interact with each other will facilitate APS’s transition to a smarter grid.

Engineers will manage the program using a high-speed application programming interface system that will serve as the nerve center of what will amount to a “virtual” rate laboratory.

As part of its effort to modernize APS’s rate structures, the study will demonstrate which price signals best encourage customers to shift their energy use to better align with APS system requirements. Participants will be able to control their home energy use from personal devices such as smart phones or tablets, all of which will give consumers an easy and low-impact customer experience.

APS officials see SIS as a positive step necessary to move toward a future that includes modernized rate structures and good pricing signals that drive market innovation and technological development.

Initiatives such as these and others in the future will give customers more control over their energy, spur local market innovation and allow APS to achieve more flexibility to integrate new energy solutions into its ‘smartening’ grid.

Going forward, APS believes programs like the Solar Partner Program and the Solar Innovation Study will be a huge win for customers, the market and the electric utility industry.

Sanket Adhikari, Hillary Waterman and Kari Rekoske contributed to this article.

Technicians from Chandler, Arizona-based Harmon Solar install panels on the home of Gregg Dufort, of Phoenix, Jul. 10, 2015. Dufort’s is the first of 1,500 rooftops to receive free solar panels through the APS Solar Partner Program, under which APS will pay customers $360 per year for 20 years.

About the Author

Ray Brooks, Arizona Public Service
Ray Brooks leads the Renewable Energy Program policy and compliance team at Arizona Public Service and spent the past year as the company’s spokesman for solar, renewable energy and political issues. A lieutenant colonel in the U.S. Marine Corps reserve, Ray’s experience also includes two tours in Afghanistan, a congressional fellowship with the U.S. House of Representatives Committee on Foreign Affairs and extensive policy experience in the Middle East and Asia Pacific regions.