A few questions back I said that there were endless ways to use solar energy. That's true, but, to make things easier to think about, all those ways can be divided into two basic categories. First, we can use the sun's energy to heat things--our houses, the water in our houses, the food in a solar cooker, and so on. This is called solar thermal energy. (Thermal means heat, so solar thermal energy just means heat energy from the sun.) The second basic way we can use solar energy is to turn light from the sun directly into electricity, using solar panels. This is usually called photovoltaics, and we?ll talk more about it in the next question.

Solar electric panels are made up of something called silicon, the same thing that makes up sand. There is more silicon on the planet than almost anything else. Even though you can find silicon almost everywhere, making a solar panel is difficult and expensive. The silicon has to be heated to super high temperatures in a big factory, and then formed into very thin wafers. When sunlight hits a solar panel, it makes electrons in the silicon move around. (Electrons are teeny tiny specks--they?re way too small for us to see, even under a microscope.) The electrons flow through wires that were built into the solar panel. And presto! We have electricity! We can do whatever we want with this electricity, run a calculator, a CD player, or, if we have big enough solar panels, a satellite! [Solar panels are also called photovoltaic panels. "Photo" means light and "voltaic" means electricity.]

When the sun stops shining on your solar panel, its electrons stop moving and electricity stops flowing. So what do you do if you want to be able to read or watch television at night? (But you don't watch television, do you?!) What stores the electricity in a flashlight? Right! A battery! And that's exactly what people do with solar panels... they attach batteries. The batteries are big, heavy, rectangular boxes, sort of like car batteries. Electricity from a solar panel flows into attached batteries while the sun shines, and then the stored electricity in the batteries can be used at night, or when the sun is behind the clouds.

Have you seen those big orange signs along the highway with flashing messages about an exit being closed or a traffic jam ahead? Ever look on top of those signs? Yup, there are big solar panels up there. Those solar panels are attached to batteries, so the signs stay lit at night. Little solar panels are used on solar calculators (the panel is usually in a little strip across the top). You might have seen solar panels on people's roofs or on poles in front of their houses. And how about in space? You know those flat, black "wings" that stick off satellites and space stations? Yeah, solar panels! Can you think of other places you?ve seen a solar panel, or that would be a smart place for one?

That depends on how much electricity you use in your house, and how much sun shines where you live. You need a lot of panels to power a whole house, though. The first thing you'd have to do, before you "went solar," would be to cut down on your electricity use. Otherwise you'd need so many panels that you'd go broke buying them and you'd have no room to play outside, because everything would be covered in solar panels. The average size of a solar system that completely powers a house is 2,000 watts. That's not a lot of power--just enough to run 20 or 30 light bulbs. (Except that if you were going to run your house off solar panels, you wouldn't use the same old light bulbs you've been using, that mostly make heat and just a little bit of light. You'd use super-efficient, long lasting florescent light bulbs.) To make 2,000 watts of power you need solar panels that are about 24 feet long by 10 feet high. That's about as long as one and-a-half cars and a little higher than most of your ceilings. That's still a lot of panels. But once you get them up, they last for well over 20 years. And since they have no moving parts, they almost never break.

Have you ever thought about what happens when you flip on the light switch and the light turns on? Electricity is flowing to the light bulb, but where does it come from? In the U.S., most of our electricity is made in giant power plants that burn coal or natural gas, or are powered by large dams or nuclear energy. That electricity sometimes travels a long way from the power plant to your home, often hundreds of miles. If you live in the country, you?ve probably seen giant towers with lots of wires running between them. (If you go close to them, you can sometimes hear them humming--that sound is being made by electrons running through the wires!) From the wires on those giant towers, electricity travels to smaller power lines on poles, which often run along roads or streets. If you live in a city, electricity probably arrives at your house in wires that run underneath the sidewalk.

If you're not using a light, or if you're not using the computer, or if you're not watching the TV, turn them off!! Riding your bike or walking or taking the bus or the train instead of getting in a car and driving somewhere saves a lot of energy, too. So does taking shorter showers (that makes me very sad, because I love long showers). Once you start trying to save energy, you'll find that there are some things in your home that you might not have to use at all. Instead of using a clothes dryer, for example, you can dry your clothes outside in the sun. Instead of playing a computer game, you can go do the dishes. (Just kidding! Sort of.) Eating food from your own garden, or food that's grown and put into packages near your home also saves energy. (The label on the package usually says where the food comes from.) Reusing and recycling things saves energy, but it saves even more if you don't use them in the first place! Once you start thinking about it, you'll find there are lots of ways to use less energy. It's even a fun thing to try and do.