Photovoltaics

Photovoltaics are any solar panels that produce electricity.

Not to be confused with solar thermal panels (non-electric) which just heat air or water in the sun.

Considerations

#Intermittency	Needs energy storage
#Rare metals	Major problem
#Need for land	Usually not a problem
#EROI	Needs improvement

Intermittency

The sun only shines during the day. Solar electricity(...)( if used as a main energy source )would need at least enough energy storage to cover a full 24-hour cycle - or maybe a bit more(...)( to make up for cloudy & rainy days, which still produce some electricity, just less^{[QUANTIFICATION needed]} ).

Energy storage is an ongoing challenge. Best candidates for solar might be iron-redox flow batteries or sodium-sulfur batteries.

Rare metals

Thin-film photovoltaics are the most common types of solar panels used today. They are relatively efficient, but depend heavily on rare 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

Need for land

Solar rooftops have enough surface area to power most of the world. Very dense cities are an exception, if they aren't surrounded by enough suburbs. Houses, if the rooftop is fully covered in solar panels, can generally produce surplus power which could power inner cities. Read more: Rooftop solar

Solar farms would be needed in cases where solar rooftops aren't enough. Some amount of land would be needed^{[QUANTIFICATION needed]}, but far less than what is used for agriculture. Solar panels would 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. (...)( There are some crops that like shade, but they could have just as well been grown in the shade of full sun crops. See: polyculture )

EROI

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.