Solar's unrealized energy potential and associated benefits are vast. In June, the International Renewable Energy Agency (IRENA) estimated that solar's share of global power generation could rise from 2 percent today to 13 percent by 2030.

What site types hold the greatest unrealized potential? Renewable Energy World spoke with a small but diverse group of industry stakeholders to identify those considered the most promising.

Some are broad-based and overlapping. Some could be considered low-hanging fruit, while others, in varying degrees, would require more resources, time and effort to develop. In addition, installing solar at decommissioned coal-fired power plants in the U.S.; along roadways, at reservoirs, and along utility and other public rights of way numbered among site types mentioned during interviews but not included below.

Across the board, contributors highlighted the importance of ongoing cost declines, performance improvements, and close cooperation between project developers, governments and communities in realizing their potential.

1. Self-Generation in Cities and Communities

Electricity consumption in cities accounts for about 75 percent of primary electricity production worldwide, according to UN-Habitat. It shouldn't be surprising then that self-generation in cities and urban areas was one of the two largely untapped, high impact opportunities cited by IRENA Resource Assessment Specialist Nicolas Fichaux.

Projects are being proposed across numerous and varied types of urban and suburban sites. The hurdles are equally varied and substantial, however; from safety issues and high installation costs to the risks of installing and operating systems where solar has never been installed before. Progress is being made, however; notably in those that have established renewable energy and/or emissions reduction targets, as well as Smart City programs.

Justin Baca, the Solar Energy Industries Association;s (SEIA) vice president of markets and research, said he would like to see more solar installed at critical infrastructure: hospitals, fire stations, police stations, emergency operations centers, emergency shelters, food distribution hubs, and communications infrastructure, such as cell networks.

“Those are places where solar plus storage can provide clean energy and services to the grid when everything is going well and prove essential when we face disruptions like natural disasters or other emergencies,” he said.

Advances are being made the world over. More than 1 GW of rooftop solar capacity is now installed in New Delhi, 533 MW of which was installed over the last 12 months, according to market research presented during this year's Intersolar Mumbai conference.

“If solar continues [to grow] in that manner, there might well be entire regions going to a grid-less business-to-consumer model operated by virtual utilities, while large utilities would then shift to a business-to-business model serving large consumers,” Fichaux said.

1a. Mid-Scale Market Opportunity: Solar for Buildings

An abundance of opportunities exists to install mid-scale, “behind the meter” solar PV (between 100 kW and 2 MW) at sites such as office buildings, hotels, warehouses and universities, according to a National Renewable Energy Laboratory (NREL) research team.

“These [building market] segments were selected based on their ability to deploy solar if key barriers, such as landlord-tenant split incentives, were removed or mitigated,” said NREL researcher and report co-author Jenny Heeter.

The potential for growth across the four building segments is substantial, even at an average 2015 installed cost of $3.20/Wdc for 50-100-kW installations, according to the NREL research team's analysis. Overall, they estimate that 44 GW could be installed, with office building PV alone accounting for half. The total could rise to more than 100 GW given cost declines in line with the 2020 targets set out in the U.S. Department of Energy SunShot Initiative.

Though not analyzed in the study, other types of city and suburban sites hold substantial promise, including K-12 schools, municipal buildings, hospitals, brownfields, and manufacturing facilities.

Installing solar PV canopies at public parking sites is another promising urban opportunity, Rocky Mountain Institute (RMI) principal and energy-built environment Ph.D. Joseph Goodman told Renewable Energy World.

“The value proposition — to site owners, businesses and utilities — has now been proven...[and] the economics are promising across the U.S.,” Goodman said “Solar at these sites can compete against retail electricity rates, and it's relatively straightforward in terms of interconnection and permitting.”

2 & 3: Beyond the Grid Edge — Solar Energy for Rural Communities and Remote Facilities

Rural and remote off-grid solar is another largely unrealized, high impact solar site type, according to Fichaux and Peter Oram, renewables sales leader for GE Power Conversion.

Mobile pay-as-you-go home and community solar energy services have taken root and are expanding across the sub-Saharan region and beyond, for example.

Larger scale opportunities are shaping up where grid power is lacking or poor. Companies with remote industrial sites are increasingly keen on solar PV as a means of assuring energy security, reducing costs, contributing to local community development and minimizing CO2 emissions and environmental impacts, Oram said.

4. Solar at the Water-Food-Energy Nexus

Climate change looms large as a motivation to install solar. Droughts are prompting government and business leaders in sub-Saharan Africa and other parts of the world to consider adding solar PV at hydroelectric power sites, for example, Oram continued.

Dispatching solar power during the day, such as periods of peak demand, opens up opportunities to hold more hydro resources in reserve so that hydroelectric generation can continue to serve as a source of baseload power, he said.

Substantial opportunities also exist to co-locate solar PV with farming and ranching in ways that yield net benefits to all involved, RMI's Goodman said. That includes use of solar-powered water pumps to build irrigation systems that use both water and energy more efficiently.

“A handful of battles over land use suggests there's room for a whole systems approach to reduce the expensive approval process and improve the value proposition to local stakeholders, including reducing the cost of electricity,” Goodman said.

5. New Opportunities: Rural Electric Cooperatives and Municipal Utilities

Electric cooperatives and municipal utilities are behind some of the most innovative solar energy programs in the U.S., Goodman said, adding that, “now that solar contracts are being structured to provide savings Year 1-25, local political debates have largely changed to discussions about how much and how fast.”

RMI sees a groundswell of bi-partisan support growing among public officials and co-op boards. Solar conforms with electric co-ops' seven cooperative principles, and municipal utilities and electric co-ops are beholden to local customers and communities rather than a separate set of investors, Goodman pointed out.

Furthermore, Public Utility Regulatory Policy Act rulings by the Federal Energy Regulatory Commission this year and last may trigger a rapid rise in purchases of local, cost-competitive solar by electric co-ops and municipal utilities.

By doing so, “they can build systems appropriate for distributed grids, while also creating local jobs and more resilient infrastructure,” Goodman said “It's a pretty exciting place to put solar.”