Update for 2016: This post is still very timely: Solar panels, mounting hardware, inverter, wiring and conduit, specialized meters: these are the components of all photovoltaic (PV) systems. But it's the solar panels themselves that not only make up the bulk of your solar quote, but whose technology, costs, and applications are the most hotly contested. People ask me all the time what the differences are among panels--here's a thorough but lay-person friendly answer.
Before the overview, a quick vocab lesson: A solar cell is the actual, wafer-thin semiconductor that produces electricity. A solar module is multiple solar cells connected to one another. A solar panel is one or more solar modules sealed up in a single object (frame) and used as part of a solar array. A solar array is the whole shebang mentioned above: the solar panels plus all the ancillary equipment necessary to not only produce electricity from sunlight, but to channel and use it.
Crystalline solar panels have been around, in varying stages of efficiency and attractiveness, for decades. They're on space stations and satellites--they're reliable and they last for ages. The semicondutor--the element responsible for the actual electric generation--is a thin sheet of crystalline silicon. There are two kinds of crystalline solar cells, which differ in how they're manufactured, their efficiency levels, and ultimately, their end cost to the consumer.
- Monocrystalline solar cells: Literally of one crystal, these cells are wafers sliced off one large, organically grown cylindrical silicon crystal. With their nearly perfect crystalline structure, these wafers are superb conductors of electricity. Current monocrystalline solar panels on the market can turn more than 15 percent of the sunlight that hits their surface into electricity.
- The crystal growth and extrusion process is time- and labor-intensive, and there is great materials loss involved in cutting the circular crystal wafers into usable octagonal solar cells, which is why solar modules made with these cells are the most expensive ones on the market.
- They are only cost-effective in some scenarios because their high efficiency means they produce more power over time than other, cheaper modules. Efficiency can go as high as 22 percent on the market, and just over 25 percent in the lab.
- You can tell when you're looking at a monocrystalline solar module because you'll see little white diamonds (or black, with some manufacturers) formed by the empty space between the edges of the octagonal cells.
- Polycrystalline solar cells: These solar cells are cut from multicrystalline silicon cast in large shallow trays. There's a lot less labor, cutting, and waste involved with polycrystalline, so they're cheaper than their mono brethren. However, the casting process produces a less perfect crystalline structure, which means these cells don't transmit energy as efficiently. These are very standard solar modules: good efficiency, decent price.
- Polycrystalline solar modules have a uniform appearance. Their color is a prettily mottled blue-black. Efficiency is usually more like 12-15 percent. These are the go-to type of solar panel for residential installations in particular, where the slight step down from monocrystalline efficiency is more than offset by the cost savings. You'll hear monocrystalline solar panels referred to as "the best"; this isn't necessarily true. Mono- and poly-cells simply have different niches.
Amorphous or Thin-Film Solar
This is the technology that's set the industry abuzz with news of ultra-cheap solar: $1/watt or less. For the record, they're talking about manufactured costs here--by no means does that number reflect what you could expect to pay as a consumer. Super cheap solar is indeed nothing more than a rumor. It's simply fact: this is high technology. High technology costs money. Federal, state, and utility solar incentives, however, can combine to lower the net cost of solar energy systems drastically, and turn a big ticket item into one excellent investment.
But yes, it's exciting that there is a cheaper solar technology afoot, and it has some great applications. Thin film can be made of different materials, sometimes silicon based and sometimes using chemical polymers as the semiconductor. Thin film drawbacks? Very low efficiency compared to crystalline, with rates typically in the 8-9 percent range; and dubious life expectancy. Crystalline solar panels are warrantied for 25-30 years, and history has proven that they can and will produce energy long after that range. Thin-film has no such tried and true promise of efficacy, which means it can be difficult to predict its returns over a longer period. It's also easier to damage. Crystalline solar modules, encased in glass and weather-sealed, are built to tough out the elements and have been proven to do so successfully. Additionally, thin film only works for solar installation sites where space is no object and angle no concern--either a perfectly tilted roof (best angle for solar is equal to the latitude of the install site) or a perfectly flat one.
So in sum:
Monocrystalline: Most efficient; most expensive; attractive octagonal design; especially good for solar installations where space is at a premium.
Polycrystalline: Efficient; moderately priced; classic "solar panel" coloration and design; best all-around choice.
Thin film: Inefficient; cheap; best for commercial installations where higher quantity can compensate for lower quality.
I hope this helps make sense of the technologies out there. If you have any questions or just want to geek out with us about cool solar technologies--like multijunction solar cells, which are a phenomenon unto themselves but pretty rare, so not included here-- just post your comments below.