Rooftop solar

Rooftops are a good place to put solar panels, because:

They are close to where the energy is typically used.
They need no extra land (unlike solar farms). That's an environmental footprint avoided.

Summary:

In most cases, rooftop solar could provide enough energy for homes, other buildings, and charging electric vehicles.
- Exception: heating in cold parts of the world
- Exception: high-density cities
Hard problems:
- Manufacturing enough solar panels without needing too many rare minerals
- Recycling solar panels at their end-of-life
- Energy storage

Potential

First let's estimate how much rooftop area is available globally. For lack of better data, let's just assume that rooftops (...)( more precisely: the surface area of all suitable rooftops combined ) are 1/4 of all "urban and built-up land" area. This assumption isn't perfect (...)( For one, there may be a lot of homes with rooftops that can't support the weight of solar panels - especially in the global south. Hopefully this problem could be mitigated with more lightweight solar panels. Well if the 1/4 turns out to be an overestimate, it still might be offset by the possibility of other suitable locations to install solar panels on built-up land. However, solar roadways are of questionable viability. ) but hopefully it's good enough.

built_up_land

1500000 km^2

Urban land, suburbs, industrial areas - global total

This is home to the vast majority of people on Earth.

Does not include most farm land.

https://ourworldindata.org/land-use

(1/4) built_up_land km^2 (calculation loading)

Assume we find a way to manufacture this many solar panels without overrunning our mineral reserves. How much energy could we generate?

solar_panel.efficiency

17%

Efficiency of an average solar panel

Some solar panels are more efficient than this, but they tend to be expensive and contain more rare metals.

sunlight_average

200 watts per m^2

Solar irradiance, averaged over a whole year INCLUDING nights, cloudy days, etc.

Note: This varies by region.
https://www.newport.com/t/introduction-to-solar-radiation
http://www.ftexploring.com/solar-energy/insolation.htm

energy.tfc

9937.70 Mtoe/year

Global energy usage - total final consumption (TFC)

Includes: fuel (80.7%) + electricity (19.3%) AFTER it is generated.

Does not include the fuel used in generating electricity. See [energy.tes] for that.

Citation: "Key World Energy Statistics 2020" IEA
- Page 47 - Simplified energy balance table - World energy balance, 2018

% energy.tfc (calculation loading)

In simple terms, this means we could just about meet the world's energy demand, with solar alone. (...)( Note: Today's energy demand is subject to change. Some factors that could decrease it: Walkability and voluntary frugalism in the western world. Some factors that could increase it: More nations becoming 'developed' in a way that copies the current western world. )

However, some of the energy might be generated in the wrong places. Low-density cities/towns may generate too much; high-density cities may not generate enough.

Locality

High-rise buildings don't have enough rooftop surface to power the whole building. But in some cases, a dense inner-city could be powered by excess energy from suburban house rooftops nearby. In what cases is this viable? Population density could be a useful measuring stick:

people per km^2 (calculation loading)

^ If your whole municipality's population density is greater than this number, you would probably need solar farms. (...)( And of course it also depends on how much energy people use; in this estimate we're assuming 2000 watts per capita, which is hopefully enough for a modern life - especially if living in a high rise. )

Charging electric vehicles

If we assume one electric car per house, we'd need to add another 543 watts per household. Note: This is power averaged over time. Peak power would be significantly higher, but that's not a big deal as vehicles could be charged during peak sunlight.

average_us_vehicle.mileage_by_time

32 miles/day

Distance driven by the average American vehicle

Top Numbers Driving America's Gasoline Demand
https://www.api.org/news-policy-and-issues/blog/2022/05/26/top-numbers-driving-americas-gasoline-demand

electric_car.efficiency

100 miles per 34.6 kWh

The "gas mileage" equivalent for an average electric car.

Average Electric Car kWh Per Mile [Results From 231 EVs]
ecocostsavings.com › average-electric-car-kwh-per-mile

li_ion.charge_discharge_efficiency

85%

When you charge a lithium-ion battery, this much of the energy can be recovered. The rest is lost as heat.

Range: 80 to 90 %
from wikipedia; haven't found original source yet

watts (calculation loading) Would the average suburban rooftop be able to provide this?

watts (calculation loading) Yes, seems doable. Obviously it depends on the size of the rooftop (in this example, we assume 1000 square feet). (...)( If we factor in all the car owners in dense cities, it might become less viable. Good thing dense areas are good candidates for walkability. )

Energy storage

So far we've looked at watts averaged over time, but actual raw watts are much higher during the day, and almost zero at night. We need energy storage to smooth things out.

Safety

The roof needs to be able to support the weight of the solar panels.
Installing the panels, without the proper skills and knowledge, could result in electrocution or fire hazards.
- Maybe this risk could be mitigated with enough engineering: Could rooftop solar kits be designed to be user-friendly and safe enough for non-professionals to install them?^{[RESEARCH needed]}

Challanges

Some roofs might be more suited for solar panels than others.^{[RESEARCH needed]}
Installation might need very specialized labor.^{[RESEARCH needed]}
Keeping snow & ice off the panels
Making solar panels cheaper, more lightweight, less demanding of rare minerals. This is generally at odds with having more efficient solar panels.^{[RESEARCH needed]}
Environmental tradeoffs for houses that are surrounded by tall trees

Combined photovoltaic and thermal panels

Solar thermal panels use the sun to directly heat water & air, without electricity as an intermediate. There are some designs that integrate with photovoltaics, to really make the most of the sun for both heating and electricity. But in which weather conditions is this viable - would it still work in colder winters?^{[RESEARCH needed]} How does it compare to pure photovoltaics combined with heat pumps?

Factories

Factories make up a significant share of global energy demand - more than homes and other buildings. Factories could also mostly run only during the day to make use of peak solar energy without much need for energy storage. But could a factory's rooftop provide most of energy needed to power the factory?^{[RESEARCH needed]}

If not, there may be other solutions:

If the factory is near a city with many houses producing excess energy from their rooftops during the day, the factory could use that as an energy source.
If the factory is far from cities, it might happen to be in an area where wind power is viable.
Perhaps the factory could scale down and decentralize production. Beware of caveat: (...)( This could easily be overall worse for the environment if existing factory equipment has to be abandoned in favor of smaller equipment to be built from scratch. But there are probably some cases where this wouldn't have to happen. )