The core of the PV technology continues to be the crystalline solar cells (mono-crystalline and polycrystalline). There are other types of photovoltaic cells, but the crystalline technology is by large the most common.
The iconic photovoltaic modules – involving groups of interconnected solar cells covered with glass and backed with a polymer or a glass sheet – are based on crystalline cells. They are the parts of the PV arrays that we all can see on the top of roofs or on ground-mounted racks.
Thin film photovoltaic cells can be integrated into building materials such as roof shingles, and many experts see them as a revolutionary improvement.
But they are not yet effective enough: thin film photovoltaic cells require 50-100% more area for the same electric output.
A 5 kW installed system may cost you, say, $20.000 - before possible tax credits.
See, on these issues: Photovoltaic Costs and Paybacks
Issues such as shade, the number of hours of sunlight and the angle at which the sun hits the PV array are critical for performance.
Rack mounting hardware and sun trackers, and issues such as roof mounting or ground mounting, all in close relationship with the sun's angle, can also be relevant.
Grid connected systems can feed excess PV power on the grid - typically during the day, when people are not at home, and when power is generated.
Besides, on-grid systems can draw power back from the utility company, during cloudy weather or in the nighttime, when the output is small, or there is no output. And that’s obviously advantageous. The alternative – a stand-alone system – requires an expensive backup system.
Installing a PV system to cover 20 or 30% of your electricity consumption is not a wise option. First you should consider improving your home's energy-performance, by installing high levels of insulation and high-performance windows, lighting and appliances.
In other words: you must trim your electricity needs to very low levels, and only then install a PV system able to provide most of your power needs. Photovoltaics make more sense in low-energy buildings.
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Choosing mono-crystalline or polycrystalline panels can be rather irrelevant. In other words: it’s mainly a question of price. Both mono-crystalline and poly-crystalline silicon cells are proven technologies, and we can’t consider one better than the other.
The mono-crystalline technology is slightly more efficient, but the peak efficiency of a photovoltaic panel is not necessarily a primary consideration. Try to balance price with efficiency and reliability.
Reliability – involving the manufacturing process – is now be a major factor of choice. The number of manufacturers have increased dramatically, but some are offering low-quality mono-crystalline and poly-crystalline products. Some reports confirm the use of sub-standard materials, and problems like poor insulation of the wire cables, or poor soldering and poor workmanship.
And these are worrisome signs, that should be looked seriously. Choosing a good manufacturer, expected to be in the market for many years, can be more important than choosing a specific PV technology.