This early solar charger device uses energy from the sun to recharge portable electronic devices.

AP Photo/Elise Amendola

Sunlight is a great source of energy -- if you're a typical plant. Plants use the energy from sunlight to create sugars. The energy from photons becomes chemical energy. Animals eat plants -- or eat animals that in turn eat plants -- and in this way most living things on Earth get their energy.

Converting sunlight into electricity has been a challenging problem for decades. Scientists first discovered the connection between sunlight and electricity in the 19th century. But those early photovoltaics, the fancy name for solar cells, were inefficient. They converted less than 1 percent of the energy they received from the sun into electricity.

After decades of research and engineering, we can now build solar cells that are more than 30 percent efficient. That has opened up opportunities to use solar energy in applications that wouldn't have been possible a few years ago. But just because something is possible doesn't make it practical. And solar panels are still relatively expensive compared to other ways to generate electricity. Some applications just aren't practical from a financial perspective.

Take cars, for example. A solar-powered car would be a useful tool. Imagine never having to pull up to a gas pump or plug into an electrical socket. The energy you needed would be free, courtesy of the sun. But cars are heavy and they need a lot of energy to move. Solar-powered cars tend to be lightweight, single-rider vehicles. They're flat and wide to maximize surface area. They aren't very fast and they have a limited range -- they won't go far in cloudy conditions or after it gets dark. So while it's possible to power a car with the sun it's still not practical.

That doesn't mean solar power is useless or that we should abandon it. There are several applications that make sense. Some require a hefty investment up front but have the potential to pay off financially in the long run. And there's no denying that solar power is more environmentally friendly than systems that emit greenhouse gases.

Up next, let's look at ways you can use solar energy.