Can Solar Rooftops Power the World?

Growing the world’s renewable energy capacity has always been a delicate balancing act, and one of the many plates we have to spin during that act is land use. Utility-scale solar and wind parks are rapidly expanding in size and prevalence all over the world, including right near my own backyard.¹ But it’s not just NIMBYism that complicates the creation of clean energy — it’s serious environmental and social concerns, too. When are the negative impacts of siting these massive projects worth the benefits? When are they not?

With these questions in mind, it’s tempting to wonder: How much solar power could we squeeze just out of rooftops? Could rooftop solar single-handedly provide us all the energy we need while avoiding legal hassles and ecological consequences? At the end of the day…do we have enough space for solar?

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We’ve discussed a lot of fascinating possibilities for increasing and improving solar power on the channel from more simple modifications to the downright fantastical. But if you’ve been watching for a while, you might have noticed a pattern in the tech I cover: the most persistent barrier to widespread use is usually cost (at least, for now). In the case of going all-in on rooftop solar, it’s not so simple.²³ Instead, it’s a matter of priorities.

Let’s step back for a second, though, and talk about why you’d want to pitch a “cover all the roofs!” approach in the first place. Utility-scale solar is definitely preferable to coal, but there’s many more considerations at play than just aesthetics when breaking new (old) ground. For one thing, as they’re being built, solar farms can soak up a lot of water — thousands of acre-feet’s worth. In some areas, that means these construction projects threaten the water supply for locals who rely on aquifers, which are difficult to monitor and preserve.⁴ Unfortunately, the same areas that have lots of sunshine to spare also have limited water resources to live.

Then there’s the other ways surrounding life is disturbed: construction noise, dust storms, glare, and of course, displacement of wildlife.⁴ So, when an undertaking like the Aratina Solar Center involves the destruction of state-protected Joshua trees in California, it’s not unreasonable to question whether going forward with large-scale construction is the best decision.⁵⁶ And by no means is this the only solar farm facing opposition from neighboring communities and conservationists…trust me.²³

Even when these proposals successfully clear the tangles of bureaucracy and public comment, it still takes a long time to get them online. This is especially true in the U.S., which is currently suffering from a grid connection backlog longer than a CVS receipt. Before utilities can move forward with their plans, they need to undergo an impact study. Off they go into what’s known as the “interconnection queues.”⁷

Over 95% of the projects idling in these interconnection queues are for renewables, with solar representing the vast majority of them. And by the end of 2023, the combined generation capacity of projects waiting for an assessment amounted to nearly 2.6 Terawatts, which is more than double the U.S.’ existing generation capacity…in total.⁸ Not all of these solar hopefuls make it to completion — in fact, most don’t.⁷ So yeah, you could say we’re a bit behind.

However, rooftop solar can easily skip over a lot of these obstacles. While I can tell you from personal experience that investing in solar panels definitely doesn’t spare you from permitting hell, it’s not like you have to get in line for an impact study. You’re taking advantage of space you already have to work with rather than carving out more land. And you can use the power you generate onsite, so you’re not at the mercy of transmission lines.⁹

So, what if we went all-in with rooftop solar? How much of our energy supply could come just from rooftops alone?

Well…I can’t tell you. Not right off the bat, anyway. Simply hypothesizing what we possibly could generate with rooftop solar can’t be adequately diluted to a single statistic, even when you apply limitations, like only covering a specific region.¹⁰ There’s just too many variables to account for, so it’s no surprise that researchers have been trying to answer this question for years.¹¹

I can’t give you a meaningful back-of-the-envelope figure for how much rooftop solar has to offer on a fully realized scale. What I can do, though, is break down how researchers have been tackling the subject — and that would be through various measures of potential. Yup…even the concept of “potential” itself has multiple subtypes in terms of calculating what we can accomplish with renewables.¹² As a result, you end up with different values depending on what kind of potential you’re looking at.¹⁰

If you imagine a pyramid of potential, like the U.S. National Renewable Energy Laboratory (NREL) has here…¹²¹³

…then the foundation begins with the physical constraints of rooftop solar. How much radiation is hitting a given rooftop in the first place? Of course, the total amount of radiation that a surface receives currently can’t come anywhere close to the energy that it ultimately produces, and that’s not just because of a solar panel’s Shockley–Queisser limit.

That brings us to the technical potential of solar: what is a particular system capable of in a particular environment, and how well does it perform? Here’s the thing: as the NREL defines it, technical potential estimates what you can physically deploy “without regard to market, economic, or policy constraints.”⁹ In other words, laying out a technical potential figure for rooftop solar doesn’t provide a full picture. We still have to contend with how costs, policies, regulations, and the public response restrains what it can actually do for us in the real world.¹⁰¹² To PV or not to PV; that is the question.

For an example of how the differences between the potential and the possible shake out, we can look at a 2016 study published by researchers from Yonsei University in Seoul, Korea. The research team zeroed in on the capital’s Gangnam district (yes, that one), and their analysis followed a three-step process of determining the area’s physical, geographic, and technical potential. Within this particular study, “physical” refers to “total solar radiation on the rooftop,” “geographical” equates to “available rooftop area for solar PV installation,” and the technical potential is the final electricity generation. Here’s a summary of results, which represent annual values:

Note that “geographic potential” is expressed in meters squared, not megawatt hours, because it represents the average amount of rooftop area compatible with solar panels.

So, you can tell right away that there’s a big gap between the physical and technological potentials. As the researchers write, “only 12.17% of the physical potential can be generated as electricity.”¹⁰ These results don’t mean rooftop solar isn’t worth it, though. To put that number into perspective, CEIC Data estimates Seoul’s all-time highest use of electricity between January 1997 and May 2018 fell within August 2016, the same year as the solar potential study’s publication. That number was an average 4,852.558 kWh mn…or 4,852,558 MWh.¹⁴ Just like that, rooftop solar in one of Seoul’s 25 districts could cover a quarter of the entire city’s energy needs. That’s impressive.

However, my point is that the study shows how establishing the suitability of rooftop solar is a layered and laborious task, even for a single region. That’s not to mention all the other circumstances that can make or break the viability of rooftop solar…including the quality of life considerations that drive its acceptance by the people it’s meant to serve (which we went over earlier). It’s not just about solar’s literal star power. Researchers also have to take into account factors like…

• How many rooftops in an area are flat?
• How many have gables?
• How many are shaded by neighboring buildings?
• How will the panels be spaced?
• What angle are they placed at?
• What’s their efficiency?
• What about the performance ratio — the difference between the electricity generated and what gets lost along the way?
• How quickly will they degrade?
• What temperatures will they be exposed to?
• What about the accumulation of dust or snow?

Basically, you can’t avoid making assumptions…and depending on what kinds of assumptions you make, your theoretical solar potential can vary wildly. That’s how estimates developed by one research team can differ from others in orders of magnitude. You can see how pronounced these differences are in this graph from a review of rooftop solar potential studies:¹¹

So, it’s hard enough collecting and refining the approximations necessary to make an educated guess about solar potentials. But no matter how precise your data is, theoretical plans can’t predict the future or anticipate every community, government, or individual decision. You might discover the ultimate solar site…but not have any policy incentives available to help do something with it. NREL pretty much sums it up in a study about community solar published earlier this year:

“Realistically, the potential accrual of benefits is a fraction of those high-end estimates based on technical potential capacity.”⁹

Where do we go from here? The truth is, weighing the trade-offs between utility-scale solar farms and a widespread rooftop offensive might be missing the forest for the trees…or maybe the sun for the rays. Because so much of solar’s efficacy is location-dependent, it doesn’t make sense to broadly advocate for one over another, and we don’t have to stop at either. Panels can infringe on the ecosystem they’re installed in, but they can also directly enhance it.

Agrivoltaics, for example, enables us to create a symbiotic relationship between farming food and solar generation. Panels can protect crops from the elements, reduce water usage, provide shade for animals, and act as hubs for attracting pollinators — which happens to be(e) the most common application of agrivoltaics in the U.S.¹⁵¹⁶ I’ve got a whole deep dive video on the practice if you’re interested.

There’s also practices like solar grazing, or letting livestock loose on fields full of solar panels and having them go to town. By partnering with sheep and cows, we can cut down both overgrown lawns and maintenance costs as they take over the otherwise expensive role of groundskeeper.¹⁷¹⁸¹⁹ If the concept of solar-over-soil gives you the warm fuzzies, I’ve gone into more detail about augmenting agriculture this way on the channel before, so you know what to do.

Now that we’ve gone over the land, what about the water? Let me float an idea by you: floatovoltaics, or solar panels that catch current in more ways than one. Any kind of water will do, from the open ocean to lakes. But we can get some of the same flexibility enjoyed by rooftop solar by integrating panels into all sorts of architecture…including irrigation canals and dams.

A while back I took a deep dive into the flow of canal floatovoltaics projects from India to California. Since then, similar efforts have kicked off in Oregon and Utah in the U.S. earlier this year.²⁰ Just this summer, the Philippines inaugurated its largest PV irrigation system yet, the first built over a canal.²¹

And if you were around for my video on small hydropower, you might remember our discussion of one particular hydro titan: the Itaipú Dam along the borders of Brazil and Paraguay. The dam already provides about 90% of Paraguay’s electricity and 15% of Brazil’s.²² In a study published in May, consulting firm PSR argues that the hydroelectric plant’s existing installed capacity of 14,000 MW could be nearly doubled by setting aside just 10% of its reservoir’s surface area for floating PV.²³

That’s not to mention the sunny spectrum of solar niches like sound barriers, balconies, billboards, fences, footpaths…heck, even monuments. By 2025, Amsterdam will allow for solar panels on roofs within protected areas of the city.²⁴ How’s that for “in my backyard”? Should we come up with a new term? IMBYism?

Once you start to examine how social and economic influences can help or hinder solar’s technical potential, it quickly becomes clear that our main conflict isn’t space. It’s a deceptively simple plan to cram as many solar panels onto as many rooftops as possible. It’s much more difficult to assess priorities and anticipate impacts on a case-by-case basis. But if solar’s massive spike in deployment, long-term drop in cost, and overall technological maturity has amounted to anything, it’s that we’re not at all short on options. So yes, we do have space for solar that doesn’t require clear cutting large swaths of land. Dual use implementation looks like it’s the key.