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Photovoltaic solar panels

Photovoltaics (PV) are any solar panels that produce electricity.

Status quo

As of 2023, solar provides about 1% of the world's energy (5% of electricity). If we want to phase out fossil fuels, then solar might need to scale up by over 50x. This comes with challenges.

Scarce metals

Major problem

Thin-film photovoltaics are the most common types of solar panels used today. They are relatively efficient, but depend heavily on scarce minerals. There's simply no way they could be scaled up enough to replace fossil fuels.
Read more: Solar panel minerals

Solution: Find some other photovoltaic tech based on more abundant minerals. Sacrificing some efficiency is okay, for the sake of making cheaper, more scalable solar panels.
Read more: solar/challenge 1


Possible problem

Status quo: Most solar panels are not recycled. Existing recycling plants can't recover very much of the rare metals in the panels. [ELABORATION needed]

As mentioned in the section above, we'll need to use some alternative solar panel tech anyway (one that is less dependent on scarce minerals). Whichever tech we choose, there will still be some metals involved, so it's more important than ever to make sure it can be recycled in a way that recovers those metals.


Short-term fluctuation: Solar panels only produce electricity during the day, not at night. Cloudy & overcast days still produce a generous amount of power, although slightly less. Days with heavy rain produce less.[QUANTIFICATION needed]

Long-term fluctuation: Solar panels produce less energy in the winter than in the summer.[QUANTIFICATION needed / case studies]

So if solar were to be scaled up enough to phase out fossil fuels,

  • We'd need at least enough battery energy storage to cover a full 24-hour cycle - or maybe a bit more.
  • Some excess summer daytime energy may be worth storing by producing hydrogen gas (which could be burned in the winter for heating).
  • To reduce the need for energy storage, it would help if people were to charge their electric vehicles mostly during the day, and if factories were to run only during the day (and maybe even temporarily shut down in the winter). [new economics needed]

Energy in production

Fair / needs improvement

Energy return on investment: about 7.5.

Solar panels are estimated to have an "energy payback" of about 4 years.[citation needed] In other words, for all the energy it takes to manufacture a solar panel, the solar panel will generate the same amount of energy after about 4 years of being installed. Since a solar panel is expected to last about 30 years, this comes out to an EROI ratio of 7.5.

4 years is a long time, which means that switching to solar might need a lot of fossil fuels to "get the ball rolling". A shorter energy payback time is a worthwhile goal, mentioned in: solar/challenge 1. This becomes even more important if there turns out to be a lot of energy losses in energy storage.

Land usage

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.

(1/4) built_up_land
Surface area of all rooftops in the world
Quick estimate based on the assumption that about a quarter of all "urban and build-up land" consists of rooftops or something else suitable for solar panels.
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.
200 watts per m^2
Solar irradiance, averaged over a whole year INCLUDING nights, cloudy days, etc.
Note: This varies by region.
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

In the simplest average case, solar rooftops are all that would be needed.

rooftops_area * solar_panel.efficiency * sunlight_average % energy.tfc (calculation loading) If all rooftops in the world were covered with solar panels, the energy produced is just about equal to today's global energy demand.

But of course life is more complicated:

  • Geography: Some regions may need more energy than the local rooftops can provide, while other regions may be the opposite. Power lines can only reach so far.
    • Countries with extreme temperatures need more energy for heating and cooling.
    • Local industry is a major part of energy demand, but it doesn't correlate neatly with local rooftop area.
    • Very high-density cities may not have enough rooftop space per capita. Sides of buildings have limited sun when they're obscured by other buildings.
      • This isn't a problem for most North-American cities, as the surrounding suburbs (low density) could produce enough surplus power for the inner city (high-density). [CASE STUDIES needed]
  • Inequality: Most of the world today is in poverty. If every nation was developed, the demand for energy would be a lot higher than the status quo used in the calculation above.
  • Lower efficiency: Since conventional solar panels are too mineral-intensive to scale up, we need some alternative which will probably be less efficient at converting sunlight into electricity.

But there is at least one factor that makes this easier:

In general, rooftop solar can almost always provide enough electricity for a house (and even charge an EV in many cases), but not always enough heating.

For heating: Burning hydrogen gas (generated from wind power) could probably make up the difference.

Solar farms may be needed in cases where solar rooftops aren't enough. The amount of land required would be far less than what's used for agriculture. [QUANTIFICATION needed] Solar panels should best be placed in areas with no fertile soil. They don't play well with agriculture, because (unlike wind power) solar panels block the sun that plants need to grow.

And although there are some crops that like the shade, they could have just as well been grown in the shade of full-sun crops instead. See: polyculture.

Compared to wind power, solar is far less land-intensive (...)( about 50x less, based on the fact that the atmosphere converts about 2% of the sun's energy (that hits the Earth) into winds ) but far more invasive to the land it does use. So rooftop solar should come first, as much as possible.

See also