Parking Lots, Rooftops, & Farms: Mapping Oʻahu’s Solar Potential

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Any discussion of solar power on Oʻahu has to begin with the amount of electricity the island actually needs in a fully electrified system. Earlier analysis stripped out aviation fuel for overseas flights, fuel bunkered for ships leaving the islands, and military fuel use. It also electrified transportation, buildings, and industry. What remains is the energy required to run the civilian economy on Oʻahu. When combustion losses are removed and efficient electric technologies replace gasoline engines and gas burners, the island’s electricity demand settles in the range of 6,000GWh per year. That number is the right target for renewable energy planning. The older numbers for petroleum consumption or total primary energy are not relevant because most of those flows were simply wasted heat from combustion engines and power plants.

The solar resource on Oʻahu is unusually strong for an electricity system that relies heavily on photovoltaic generation. The island sits near 21° north latitude and receives consistent sunlight throughout the year. Day length varies from roughly eleven hours in winter to about thirteen and a half hours in summer. Solar capacity factors for fixed rooftop systems generally fall between 18% and 20%. Utility scale systems with single axis tracking often reach around 23%. These numbers translate easily into annual energy. A 1MW solar installation operating at a 20% capacity factor produces about 1.75GWh of electricity each year. Multiply that by the number of megawatts installed and the annual energy becomes straightforward to estimate.

Formal solar resource assessments already provide a baseline for the island. Work summarized by the Hawaiʻi Natural Energy Institute and based on National Renewable Energy Laboratory land use screening identified roughly 1,862MW of potential utility scale solar capacity on Oʻahu after excluding wetlands, protected lands, steep slopes, and other unsuitable areas. At a capacity factor of about 23%, that level of capacity would produce around 3,700 to 4,000GWh per year. That number is significant. It represents roughly half of the electricity required in the electrified Oʻahu economy. But it is only one part of the solar picture because it focuses on open land installations.

Rooftop solar is the second large category. Hawaiian Electric reports that nearly half of single family homes on Oʻahu already have rooftop solar systems installed. That level of penetration is remarkable by global standards and demonstrates both the quality of the solar resource and the economic attractiveness of rooftop systems in Hawaiʻi. However, the existing installations are concentrated in single family neighborhoods. Large opportunities remain on commercial roofs, warehouses, schools, government buildings, and multifamily housing complexes. A conservative assumption is that several hundred megawatts of additional rooftop capacity could still be installed on buildings that have suitable roof structure and exposure. If another 600MW of rooftop solar were deployed with an 18% capacity factor, it would produce roughly 950GWh per year. That contribution alone would supply more than 10% of the island’s electrified electricity demand.

The largest overlooked solar category on Oʻahu is parking canopy solar. Most technical potential studies focus on rooftops and open land. Parking lots are often ignored even though they cover large areas in automobile oriented cities. Oʻahu has about 792,000 registered vehicles according to the Hawaiʻi Department of Business, Economic Development and Tourism. A typical urban planning assumption is roughly 2.5 parking spaces per vehicle across residential, commercial, and institutional uses. That suggests close to two million parking spaces across the island. Each parking space including circulation lanes occupies roughly 30 square meters. Multiplying those numbers produces nearly 60 square kilometers of parking surface area. Not all of that area is suitable for solar canopies, but covering even 40% of those surfaces would yield around 24 square kilometers of canopy structures.

The National Renewable Energy Laboratory estimates that parking canopy solar installations can achieve about 183MW of capacity per square kilometer. Applying that density to 24 square kilometers of canopy area produces roughly 4,350MW of installed capacity. At an 18% capacity factor that capacity would generate about 6,900GWh per year. That single category could produce more electricity than the entire electrified Oʻahu economy requires. Even if the canopy coverage assumption is cut in half, the resulting generation still reaches roughly 3,400GWh per year. Parking canopy solar stands out as the largest untapped solar resource on the island.

Parking canopies also deliver benefits beyond electricity generation. Shade reduces vehicle interior temperatures, which matters in a tropical climate where parked cars heat rapidly. Canopies also provide covered walkways for pedestrians and protect vehicles from weather. Because many parking lots are located near retail centers, office buildings, and transit stops, canopy structures provide natural locations for electric vehicle charging infrastructure. In a city where cars dominate daily travel, it is surprising how little canopy solar has been deployed. The absence is notable because it addresses multiple urban challenges at once. Solar generation, heat island reduction, and EV charging infrastructure can all be delivered from the same structures. I’ve driven on Oʻahu and can attest to the heat of cars left in the sun and the sheer amount of parking everywhere.

Agrivoltaics provides another layer of solar opportunity. Agricultural land on Oʻahu faces competing pressures from development and water constraints. Dual use solar installations allow crops and photovoltaic panels to share the same land. Some crops benefit from partial shading because it reduces water loss and heat stress. If between two thousand and six thousand acres of agricultural land hosted agrivoltaic systems, and if those installations used the same land intensity as typical ground mount solar of roughly 7.7 acres per megawatt, the island could support between 260MW and 780MW of additional capacity. At a 23% capacity factor those installations would generate roughly 500 to 1,600GWh per year depending on scale. A central estimate around 1,050GWh is reasonable.

Vertical or facade mounted solar panels add another incremental contribution. Large warehouse walls, industrial buildings, and sound barriers can support vertical photovoltaic installations. Vertical panels generate less energy per square meter than tilted panels because they capture less direct sunlight, but they produce electricity in early morning and late afternoon when the sun angle is low. A modest deployment of around 500MW of vertical solar across commercial and industrial structures could generate about 530GWh per year at a 12% capacity factor. The contribution is smaller than rooftop or canopy solar but still meaningful.

As a note, this is another area where I have to offer a mea culpa, although a nuanced one. I have been clear in the past that building integrated photovoltaic (BIPV) wasn’t a reasonable choice due to complexity of wiring, poor angles and cost. However, solar panels have become so inexpensive that the economic case has been upended for specific BIPV use cases, and enabled others. Pakistan’s massive rooftop deployment is mostly flat mounted because that’s easy and cheap. Wall mounted solar for morning and late afternoon generation now pencils out. People are building fences of solar panels because it’s cheaper than using traditional fencing material and delivers electricity. Balcony solar can actually represent 1% of Germany’s generation with reasonable projections. As I said to the Green Shipping audience in Vancouver in December, “If you aren’t paying close attention, everything you think you know about solar and batteries is wrong.”. That applies to me as well, and as the facts have changed, so has my opinion. That said, solar tiles and windows remain outside of my field of sensible solution sets.

The transition away from fossil fuel infrastructure also opens up new sites. The refinery and associated storage tanks near Kapolei occupy large parcels of flat industrial land with strong grid connections. As petroleum demand declines in the electrified economy, portions of these sites can be redeveloped. Aviation and maritime fuel supply, to be covered in a later assessment, will still require some infrastructure, but many areas currently devoted to petroleum handling could host solar installations on rooftops, parking areas, and redeveloped industrial facilities. Assuming around 300MW of solar capacity on these sites with a 20% capacity factor yields roughly 530GWh per year.

Adding these categories together illustrates the scale of the solar resource. Utility scale installations contribute about 3,700GWh. Additional rooftop systems provide about 950GWh. Parking canopy systems contribute about 6,900GWh in the central scenario. Agrivoltaics adds roughly 1,050GWh. Vertical panels provide around 530GWh. Redeveloped fossil fuel sites add another 530GWh. The combined central estimate reaches roughly 13,700GWh per year. Even a conservative version of the calculation produces more than 10,000GWh annually.

Comparing that number to Oʻahu’s electrified electricity demand clarifies the situation. The island’s economy requires roughly 6,000GWh of electricity per year in the electrified scenario. The central solar estimate exceeds that demand by a wide margin. That does not mean every megawatt of potential solar would be built. It means that the island has enough suitable surfaces to produce more solar energy annually than it consumes.

The difference between annual energy potential and practical electricity supply lies in timing. Solar panels generate electricity during daylight hours, with the largest output around midday. Electricity demand often peaks in the evening when solar generation falls. Batteries provide the bridge between those periods. Storage systems charge during midday when solar output is high and discharge during evening hours when demand rises. Battery installations operating for four to eight hours can shift a large portion of daily solar production into the evening.

One design choice that becomes increasingly important in a solar heavy system is the orientation of panels. Traditional rooftop systems in temperate regions often face south to maximize annual output and concentrate production near midday. On Oʻahu, that strategy is less useful because midday solar production will already be abundant. A better approach is to deliberately split installations between east facing and west facing panels. East facing panels begin producing earlier in the morning, while west facing panels continue generating later into the afternoon and early evening. Each orientation produces less total energy than a perfectly south facing system, but the generation profile becomes much broader across the day. Instead of a sharp spike at noon, the system produces a wider plateau of generation from morning through late afternoon. When this approach is applied across thousands of rooftops and parking canopies, the aggregate effect is significant. The midday peak is reduced, solar output remains stronger during the shoulder hours when demand is rising, and the amount of battery storage required to shift energy into the evening declines. This is such a dominant pattern that I observed it in the Netherlands at a GW scale hybrid wind, solar and battery farm.

The low variability of solar output on Oʻahu also supports a solar dominated system. The island’s weather patterns are dominated by trade winds that produce moving clouds rather than persistent overcast conditions. Cloud cover can reduce output for minutes or hours but rarely for many days. Batteries and flexible loads can manage these short term fluctuations. Large seasonal swings like those seen in higher latitude regions are much smaller in Hawaiʻi.

Even with these advantages, relying exclusively on solar would create vulnerabilities. Storm systems and unusual weather patterns can reduce generation for several days. A resilient system benefits from diversity. Onshore wind, though limited on Oʻahu, can contribute several hundred gigawatt hours per year. Offshore wind may add more in the future. Demand management, electric vehicle charging control, and water heating storage can shift loads into periods of high solar output. Biomethane will be explored as well.

Despite these caveats, solar is likely to dominate Oʻahu’s renewable energy future. The island receives abundant sunlight, and photovoltaic technology continues to decline in cost. When panels are inexpensive enough, installing them on parking structures, building walls, and other unconventional surfaces becomes economically attractive. Combined with batteries and flexible demand, solar generation can meet the majority of the island’s electricity needs.

The numbers support a clear conclusion. Oʻahu does not lack solar potential. The island has more than enough suitable surfaces to generate the electricity required for its electrified economy. The challenge is not finding sunlight. The challenge is building the infrastructure, storage systems, and grid management capabilities required to convert that sunlight into reliable electricity.

That infrastructure just keeps on generating, unlike LNG which requires a steady stream of tankers. The choice is clear. Just as Pakistan put in place 32 GW of new solar mostly on rooftops in the past two years and is now turning away full LNG tankers, if Hawaiʻi opts for LNG, it will end up with long term contracts for LNG it doesn’t need.