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Photovoltaics (PV) work by converting the sun's energy into electricity using roof-mounted panels. Cheaper units convert some 5 per cent of solar energy into electricity and more efficient, and more expensive units, convert up to 18 per cent of energy received into electricity.

Power output depends on the type of materials used in construction and the amount of sunlight received. The maximum output from PV systems is in the summer, but the maximum power usage in a property is in mid-winter. Energy from these systems can be sold back to the National Grid.

A maximum output energy rating for panels can be misleading and does not relate to the amount of resource (sunlight) that is actually available at any one location. A better way of comparing panels is their % efficiency; this tells you what % of the light landing on them they will turn into electricity. This combined with a figure for the average solar radiation at your site and the size of the panels will give you a far better idea of how much electricity you will generate. Expected annual energy yield (kWh) = Panel Efficiency (%) X Average annual solar radiation (kWh per m²) X Area of panels (m²).

The cost of 1kW Mono-crystalline PV solar panels fully installed starts from £7,979. A typical low-energy house will need 3kW to 4kW capacity.

Quick Facts

As a general rule a 1kWp photovoltaic array, measuring at least 7.5m2, will generate approximately 750kWh per year in the UK climate.

Carbon dioxide savings for 1kWp will be in the order of 480kgCO2 per year.

A common rule of thumb is that average power is equal to 20% of peak power, so that each peak kilowatt of solar array output power corresponds to energy production of 4.8 kWh per day (1kWp x 24hours x 20%).

A typical low-energy house will need 3kWp to 4kWp capacity.

Types of PV

Mono-crystalline Silicon Cells:
Made using cells saw-cut from a single cylindrical crystal of silicon, this is the most efficient of the photovoltaic (PV) technologies. Mono-crystalline cells have high efficiencies, typically around 15%, although the manufacturing process required to produce mono-crystalline silicon is complicated, resulting in slightly higher costs than other technologies.

Poly-crystalline Silicon Cells (multi-crystalline):
Made from cells cut from an ingot of melted and recrystallised silicon. In the manufacturing process, molten silicon is cast into ingots of polycrystalline silicon; these ingots are then saw-cut into very thin wafers and assembled into complete cells. Poly-crystalline cells are cheaper to produce than mono-crystalline ones. Average efficiencies of around 12%.

Thick-film Silicon:
Another multi-crystalline technology where the silicon is deposited in a continuous process onto a base material giving a fine grained, sparkling appearance. Like all crystalline PV, this is encapsulated in a transparent insulating polymer with a tempered glass cover and usually bound into a strong aluminium frame.

Amorphous Silicon (thin film):
Amorphous silicon cells are composed of silicon atoms in a thin homogenous layer rather than a crystal structure. Amorphous silicon absorbs light more effectively than crystalline silicon, so the cells can be thinner. For this reason, amorphous silicon is also known as a "thin film" PV technology. Typical efficiencies of around 6%, but they are easier and therefore cheaper to produce.

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