What’s Current? Issue #31 – The Potential of Rooftop Solar

California’s central planners are determined to stay ahead of the entire world when it comes to renewable energy and achieving “net zero.” It is an expensive and intrusive experiment, and we’re the lab rats. But that doesn’t mean every renewables innovation is bad. And for the hardened skeptics, we can put it another way: At the very least, some solutions are less bad than others.

In that spirit, an examination of photovoltaics is worth considering, in particular if they are hosted on rooftops. To begin with, apart from, maybe, geothermal energy, photovoltaics use less space than other renewables. Far less space.

Corn ethanol, a popular biofuel, is the prime example. A good yield for corn ethanol is 500 gallons per acre. Depending on the use, according to the U.S. Dept. of Energy, a gallon of ethanol contains around 80,000 BTUs. Since a kilowatt-hour is 3412 BTUs, it’s a simple matter to calculate that corn ethanol will yield 6.75 gigawatt-hours per square mile per year. Since California’s voracious appetite for electricity in 2022 was 287,000 gigawatt-hours, then even if 100 percent of the energy in corn ethanol could be converted into electricity (it’s more like 40 percent), it would take 42,000 square miles of cornfields to supply California’s electricity requirements.

Another favored renewable, of course, is wind. So how’s wind doing in terms of land required? Much better than biofuel, that’s for sure. The Tehachapi Wind Resource Area is one of California’s best sites for wind turbines. From observing the rows of 1.5 and 3.0 megawatt turbines using Google satellite imaging, they appear to average around 20 per square mile. Making this assessment is an inexact science, but the areas with larger 3.0 megawatt turbines typically number around 20 per square mile. Even if all of them had a 3.0 megawatt capacity, at a yield of 25 percent, they would generate 131 gigawatt-hours per square mile per year. That means to supply California’s electricity demand in 2022 using land based wind turbines would require 2,184 square miles.

But what about solar? Here’s where it gets really interesting. At 12.5 watts per square foot, a yield of 25 percent, and allocating 50 percent of the land on the solar farm for space between the panels, access roads and interconnections to the grid, it would only require 750 square miles of photovoltaic panels to generate 100 percent of California’s electricity consumed in 2022.

Biofuel, 42,000 square miles. Wind farms, 2,184 square miles. Solar farms, 750 square miles. When it comes to renewables, does this help you pick your poison? But the story gets better. Why not put these panels on rooftops? Is there room?

According to a NREL study conducted in 2016, there were just over 10 billion square feet of usable rooftop space in California for PV. Massive warehouse construction and ongoing construction of all types – even a few homes – since then mean this number has only increased. But 10 billion square feet, at 12.5 watts per square foot and a 25 percent yield means if 100 percent of California’s usable rooftop space had photovoltaic arrays, it would generate 273,750 gigawatt-hours, nearly equivalent to California’s entire 287,220 gigawatt-hours of electricity consumption in California in 2022.

For this reason, California’s state legislature should think twice about taking away the financial incentives for homes and businesses to install rooftop photovoltaics. Photovoltaics on rooftops don’t use up farmland, nor do they despoil any other outdoor environments. Every photovoltaic panel on a roof is one less photovoltaic panel diminishing farm output or disrupting a desert tortoise.

Rooftop photovoltaics have other virtues. They bring power directly into the urban areas where the power is consumed, dramatically reducing the need for additional transmission lines. And every one of them, presumably, will be managed and maintained by private property owners, reducing the burden on utility companies.

Another interesting virtue of rooftop photovoltaics is their potential to exist immediately adjacent to distributed storage in the form of vehicle-to-grid enabled EVs and hybrids. Note to skeptics: Even if EVs may not be there yet, hybrids are definitely ready for prime time. And once again, bidirectional charging stations in parking areas adjacent to buildings can be decentralized and privately managed and maintained. Drive your car to work, charge all day, then drive it home and let it power your home when the sun is down.

Decentralized generation, decentralized storage. Massive private investment in small-scale independent, vertically integrated power providers. Who could possibly object?

This is not the least bit infeasible. If only 10 percent of California’s 14.3 million registered automobiles (that doesn’t include any commercial vehicles) were EVs with 50 kilowatt-hour batteries, they would have a total storage capacity of over 70 gigawatt-hours. California’s goal for total utility scale storage by 2045 is only 52 gigawatt-hours.

It is unrealistic to expect renewables to replace all of California’s energy requirements. Retrofitting our fleet of natural gas power plants, drilling and refining our oil and gas in-state, expanding our nuclear energy capacity, and continuing to develop geothermal energy are all necessary if Californians are serious about maintaining an abundant and affordable energy supply, and setting an example that other states and nations will enthusiastically follow.

Moreover, the cautionary facts surrounding renewables must be acknowledged, starting with the tremendous consumption of resources it takes to build them, their relatively short service life and recycling costs, and the tremendous amount of space required to deploy them. But rooftop solar, buffered with a fleet of advanced hybrids and EVs with onboard vehicle-to-grid technology, is a combination that might well belong in the mix.