The Earth intercepts a lot of solar power: 173 thousand terawatts. That’s ten thousand times more powerthan the planet’s population uses. So is it possible that one day the world could be completelyreliant on solar energy? To answer that question, we first need to examine how solar panelsconvert solar energy to electrical energy. Solar panels are made up of smaller unitscalled solar cells. The most common solar cellsare made from silicon, a semiconductor that is the secondmost abundant element on Earth. In a solar cell, crystalline silicon is sandwichedbetween conductive layers. Each silicon atom is connectedto its neighbors by four strong bonds, which keep the electrons in placeso no current can flow. Here’s the key: a silicon solar cell usestwo different layers of silicon. An n-type silicon has extra electrons, and p-type silicon has extra spacesfor electrons, called holes. Where the two types of silicon meet, electrons can wander acrossthe p/n junction, leaving a positive charge on one side and creating negative charge on the other. You can think of lightas the flow of tiny particles called photons, shooting out from the Sun. When one of these photons strikesthe silicon cell with enough energy, it can knock an electron from its bond,leaving a hole. The negatively charged electron andlocation of the positively charged hole are now free to move around. But because of the electric fieldat the p/n junction, they’ll only go one way. The electron is drawn to the n-side, while the hole is drawn to the p-side. The mobile electrons are collected bythin metal fingers at the top of the cell. From there, they flow throughan external circuit, doing electrical work, like powering a lightbulb, before returning through the conductivealuminum sheet on the back. Each silicon cell only puts outhalf a volt, but you can string themtogether in modules to get more power. Twelve photovoltaic cells are enoughto charge a cellphone, while it takes many modulesto power an entire house. Electrons are the only moving partsin a solar cell, and they all go back where they came from. There’s nothing to get worn outor used up, so solar cells can last for decades. So what’s stopping us from beingcompletely reliant on solar power? There are political factors at play, not to mention businesses that lobbyto maintain the status quo. But for now, let’s focus on the physicaland logistical challenges, and the most obvious of those is that solar energyis unevenly distributed across the planet. Some areas are sunnier than others. It’s also inconsistent. Less solar energy is availableon cloudy days or at night. So a total reliance would require efficient ways to get electricityfrom sunny spots to cloudy ones, and effective storage of energy. The efficiency of the cell itselfis a challenge, too. If sunlight is reflectedinstead of absorbed, or if dislodged electrons fall back intoa hole before going through the circuit, that photon’s energy is lost. The most efficient solar cell yet still only converts 46% ofthe available sunlight to electricity, and most commercial systems are currently15-20% efficient. In spite of these limitations, it actually would be possible to power the entire worldwith today’s solar technology. We’d need the fundingto build the infrastructure and a good deal of space. Estimates range from tensto hundreds of thousands of square miles, which seems like a lot, but the Sahara Desert alone is over3 million square miles in area. Meanwhile, solar cells are gettingbetter, cheaper, and are competingwith electricity from the grid. And innovations, like floating solar farms,may change the landscape entirely. Thought experiments aside, there’s the factthat over a billion people don’t have accessto a reliable electric grid, especially in developing countries, many of which are sunny. So in places like that, solar energy is already much cheaperand safer than available alternatives, like kerosene. For say, Finland or Seattle, though, effective solar energymay still be a little way off.