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The Earth intercepts a lot of solar power:

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173 thousand terawatts.

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That's ten thousand times more power
than the planet's population uses.

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So is it possible that one day

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the world could be completely 
reliant on solar energy?

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To answer that question,

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we first need to examine how solar panels
convert solar energy to electrical energy.

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Solar panels are made up of smaller units
called solar cells.

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The most common solar cells
are made from silicon,

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a semiconductor that is the second
most abundant element on Earth.

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In a solar cell,

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crystalline silicon is sandwiched
between conductive layers.

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Each silicon atom is connected
to its neighbors by four strong bonds,

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which keep the electrons in place
so no current can flow.

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Here's the key:

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a silicon solar cell uses
two different layers of silicon.

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An n-type silicon has extra electrons,

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and p-type silicon has extra spaces
for electrons, called holes.

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Where the two types of silicon meet,

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electrons can wander across 
the p/n junction,

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leaving a positive charge on one side

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and creating negative charge on the other.

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You can think of light 
as the flow of tiny particles

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called photons,

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shooting out from the Sun.

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When one of these photons strikes
the silicon cell with enough energy,

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it can knock an electron from its bond,
leaving a hole.

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The negatively charged electron and
location of the positively charged hole

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are now free to move around.

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But because of the electric field
at the p/n junction,

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they'll only go one way.

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The electron is drawn to the n-side,

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while the hole is drawn to the p-side.

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The mobile electrons are collected by
thin metal fingers at the top of the cell.

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From there, they flow through 
an external circuit,

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doing electrical work,

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like powering a lightbulb,

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before returning through the conductive
aluminum sheet on the back.

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Each silicon cell only puts out
half a volt,

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but you can string them 
together in modules to get more power.

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Twelve photovoltaic cells are enough
to charge a cellphone,

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while it takes many modules 
to power an entire house.

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Electrons are the only moving parts
in a solar cell,

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and they all go back where they came from.

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There's nothing to get worn out
or used up,

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so solar cells can last for decades.

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So what's stopping us from being
completely reliant on solar power?

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There are political factors at play,

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not to mention businesses that lobby
to maintain the status quo.

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But for now, let's focus on the physical
and logistical challenges,

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and the most obvious of those

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is that solar energy 
is unevenly distributed across the planet.

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Some areas are sunnier than others.

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It's also inconsistent.

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Less solar energy is available 
on cloudy days or at night.

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So a total reliance would require

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efficient ways to get electricity 
from sunny spots to cloudy ones,

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and effective storage of energy.

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The efficiency of the cell itself
is a challenge, too.

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If sunlight is reflected 
instead of absorbed,

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or if dislodged electrons fall back into
a hole before going through the circuit,

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that photon's energy is lost.

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The most efficient solar cell yet

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still only converts 46% of 
the available sunlight to electricity,

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and most commercial systems are currently
15-20% efficient.

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In spite of these limitations,

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it actually would be possible

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to power the entire world 
with today's solar technology.

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We'd need the funding 
to build the infrastructure

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and a good deal of space.

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Estimates range from tens 
to hundreds of thousands of square miles,

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which seems like a lot,

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but the Sahara Desert alone is over
3 million square miles in area.

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Meanwhile, solar cells are getting
better, cheaper,

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and are competing 
with electricity from the grid.

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And innovations, like floating solar farms,
may change the landscape entirely.

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Thought experiments aside,

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there's the fact 
that over a billion people

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don't have access 
to a reliable electric grid,

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especially in developing countries,

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many of which are sunny.

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So in places like that,

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solar energy is already much cheaper
and safer than available alternatives,

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like kerosene.

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For say, Finland or Seattle, though,

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effective solar energy 
may still be a little way off.