Inside a solar panel there are six parts stacked like a sandwich: a sheet of glass on top, a clear glue, the silicon solar cells, a back sheet, a metal frame around the edge and a small junction box on the back.
From outside, it just looks like a flat black or blue rectangle on a roof. But every one of those parts has a job, and together they turn solar energy into electricity for decades.
So let's open it up, from the top down.
| Part (top to bottom) | What it is made of | Its job |
|---|---|---|
| Front glass | Tempered glass with a coating | Lets sunshine in, keeps hail out |
| Encapsulant | Clear plastic glue (usually EVA) | Holds the cells and keeps water out |
| Solar cells | Thin slices of silicon | Turn sunlight into electric current |
| Back sheet | Plastic sheet | Seals and insulates the back |
| Frame | Aluminum | Makes the panel stiff and easy to mount |
| Junction box | Plastic box with diodes | Where the wires come out |
The Glass: Protection For The Solar Cells
The top layer of every solar panel is a sheet of super-strong tempered glass. It has two jobs that pull in opposite directions. It has to let as much sunlight through as possible, and it has to take a beating from hail, sand and snow.
So makers use glass with very little iron in it and give it a coating that cuts down reflections. Every bit of sunshine that bounces off is energy lost, so this small design trick helps the panel's efficiency.
How tough is it? To pass the standard test, a panel gets hit by 25-millimeter ice balls fired at about 83 kilometers per hour. That is a hailstone the size of a big marble, moving faster than a car on a city street.
The Solar Cells: Where Photovoltaic Energy Starts
The dark squares you can see through the glass are the solar cells. Each one is a thin slice of silicon about as wide as your fist, and a typical panel holds between 36 and 72 of them, connected in a chain.
Silicon comes from sand. For solar cells it gets cleaned until it is 99.9999% pure, which scientists call "six nines".
Scientists also call silicon a "semiconductor". That's just a fancy word for a material that can move electricity around, but only when something gives it a push. Sunlight is the push, and this is the process that lets solar panels generate electricity with no fuel at all.
Sunlight is made of tiny packets of energy called photons. When photons are absorbed by the cell, they knock electrons loose, and those electrons start bouncing around like little balls. Thin metal lines on the cell catch them and send them down a wire. That flow is the electricity your panel will produce.
Scientists call this the photovoltaic effect. "Photo" means light and "voltaic" means electricity, so the name literally says light-electricity. The full story of solar power is on our solar page, and there is a deeper look at how solar panels work too.
The Encapsulant: The Glue That Keeps Water Out
Above and below the solar cells sits a clear, sticky layer called the encapsulant. Usually it's a plastic called EVA.
Think of it like the cheese in a grilled cheese sandwich. At the factory, the whole stack goes into a hot press, the EVA melts, and it glues the glass, cells and back together into one solid piece. It also keeps electricity in the cells from leaking where it shouldn't, and it keeps water away from the cells, which is why a panel can sit through rain and snow for years.
The Back Sheet: The Seal Underneath
Underneath the cells there's a thin plastic back sheet, white on most panels.
Its job is simple. It seals the bottom of the sandwich so moisture can't sneak in, and it insulates the wiring on the back of the cells. It also takes the knocks when the panel gets carried around or bolted down.
The Frame: The Bones Of The Panel
A strong, light aluminum frame wraps around the whole sandwich. It keeps the glass from bending and cracking in wind or under snow, and it gives the mounting clamps something to grip when the panel goes on a roof or a ground rack.
Some thin-film panels skip the frame and use a flexible edge instead. That is how you get bendy ones that can curve over a boat or a van roof.
The Junction Box: Where The Current Leaves The Panel
On the back of the panel is a small plastic box about the size of a wallet. This is where the wires from all the solar cells meet and leave.
Inside the box are bypass diodes. A diode is like a one-way valve for electricity. When part of the panel sits in shade, the diodes let the power route around the shaded cells, so one shadow doesn't drag the whole panel down.
The cables from the junction box run to the rest of your solar power system. The cells make direct current (DC), which flows one way only. A charge controller feeds that power into your batteries, and an inverter converts it into alternating current (AC), the kind your wall sockets use. Here is the difference between DC and AC power, and here are all the electronic parts in a solar setup.
Do Solar Panels Store Electricity?
No. A solar panel has no storage inside, so it only makes electricity while sunshine hits the cells. When a cloud passes, output drops, and at night it stops.
That's why an off-grid system pairs its panels with battery storage. During the day, extra power from the roof charges the battery, and at night the battery keeps the lights on. If you're curious what happens inside that box, see how home batteries work.
What's The Difference Inside Mono, Poly And Thin-Film Panels?
The basic stack is the same in all of them: glass on top, cells in the middle, a seal at the back and a junction box. What changes is the cell part.
Monocrystalline panels use smooth black cells cut from a single crystal. Polycrystalline panels use blue, speckled cells made from many crystals melted together. Thin-film panels skip the thick slices and use a very thin coating of sun-catching material instead. Each cell technology gives a different look and price, but the basic sandwich stays the same.
How Long Do Solar Panels Work For?
Here is the surprising part. A solar panel has no moving pieces and no fuel, so it just sits there and works. According to the US Department of Energy, the average panel now lasts about 25 to 35 years, up from around 20 years in 2007.
And when it does wear out, most of it can be used again. Glass makes up about 75% of a panel's weight, and both the glass and the aluminum frame are easy to recycle. Getting the silicon back out is still harder, and the industry is working on it.
Planning your own system? Find out how many solar panels you need to go off-grid, or try the solar panel calculator.
Sources
- US EIA, Photovoltaics and electricity
- PV-Manufacturing.org, PV module manufacturing
- Sinovoltaics, What is IEC 61215 MQT 17? Solar panel hail test standard explained
- Solar Daily, For solar grade silicon, how many Ns are enough?
- US Department of Energy, End-of-life management for solar photovoltaics
- US EPA, Solar panel recycling
Hard Words
| Term | What it means |
|---|---|
| Tempered glass | Heat-treated super-strong glass. The top of every solar panel. Lets light through and stops hail. |
| Solar cell | A thin slice of silicon about as wide as a fist. Each cell catches a little light and makes a little electricity. Many cells go into one panel. |
| Silicon | Refined sand, cleaned until it is 99.9999 percent pure. The main material inside the cells. |
| Semiconductor | A material that can carry electricity, but only when given a push. Sunlight is the push for a solar cell. |
| Electron | A tiny particle in atoms. When sunlight knocks electrons loose, their flow through a wire is electricity. |
| Encapsulant | The clear glue above and below the cells, usually EVA. Melted at the factory to bond everything and keep moisture out. |
| Back sheet | The thin plastic sheet on the back. Seals out moisture and insulates the wiring. |
| Junction box | The small plastic box on the back of the panel where the wires come out. |
| Diode | A one-way valve for electricity. |
| Aluminum frame | The strong, light metal frame around the edge of a panel. Gives the mounting hardware something to grip. |
| Bypass diode | A diode in the junction box that lets power route around shaded cells, so one shadow does not drag down the whole panel. |
Common Questions
What's the top layer of a solar panel?
Tempered glass with very little iron and an anti-reflection coating, tough enough to pass a hail test.
What are solar cells made of?
Thin slices of silicon cleaned to 99.9999 percent purity. A typical panel has 36 to 72 of them wired together.
What does the encapsulant do?
It is the clear glue, usually EVA, melted around the cells at the factory to bond the stack and keep water out.
Why does a solar panel have a junction box?
That is where the wires come out. It also holds bypass diodes that let power route around shaded cells.
Do solar panels store electricity?
No. They only make electricity while light reaches them. To have power at night you need a battery.
How long does a solar panel last?
About 25 to 35 years on average, according to the US Department of Energy.
Do mono, poly and thin-film panels have the same parts?
Mostly, yes. The big difference is the kind of cell inside.