Solar PV (photovoltaic) systems convert renewable energy from the sun into electricity. The electricity generated can be used within the building or exported to the grid. There are various types of PV technology, including solar panels, solar tiles, and solar film. All will produce some electricity in dull light, although they work much better in direct sunlight!
Solar electricity is generated on site, using a renewable energy source that is free and will never run out. Moreover, there are no carbon emissions. However, the PV systems are expensive to buy and install, although grants are available to help offset the inital outlay.
How does it work?
The basic unit of any PV system is the PV cell, consisting of a semiconductor material, usually silicon, sandwiched between glass panels. Light falling on the cell excites electrons in the silicon, which are conducted away from the cell through a grid of metallic conductors.
Each cell produces only a small amount of electric current, so numerous cells are connected in series to form a module. In turn, modules can be connected to form PV arrays of various shapes and sizes.
The electricity flowing from any PV array is direct current (DC). This must be converted to alternating current (AC) before it can power household applicances or be exported to the grid. This requires an inverter.
Surplus energy can be stored in purpose-built storage devices, which charge using either DC or AC, depending on their design. These units contain their own chargers, inverters, and control systems, and are usually monitored remotely via the internet. See 'Energy storage: a smart move?'
The electrical energy can also be stored in the form of heat, for example as hot water via an immersion heater or in electric storage heaters or underfloor heating. See Optimiser makes the most of solar PV.
Types of PV arrays fall into several categories, including monocrystalline arrays, polycrystalline arrays, and thin-film (amorphous) arrays.
How much electricity will I get?
Photovoltaic arrays are rated according to their maximum power output under ideal conditions, in kilowatts peak (kWp). A typical crystalline PV module measures about 1600 by 800 mm and weighs some 15 kg. It will deliver 160-170 watts peak (Wp). Each kWp requires a total module area of at least 8 square metres.
The government's standard assumption for PV performance in much of the UK is that 1 kWp of installed capacity generates 800 kilowatt hours (kWh) of electricity per year. Purpose-built domestic systems are typically rated at 2.5-4 kWp, which over a year should provide around 2000-3200 kWh, representing a third to a half of the electricity required by the typical household.
However, to achieve an output of 3kWp would require 12 to 18 PV modules, depending on their dimensions, installed in a suitable location, generally on a south-facing pitched roof. On most houses the available roof space is limited, so the number of modules that can be fitted over the existing roof is restricted.
To maximize the potential of the roof area it is best to use solar tiles or slates (' in-roof modules') instead. But these are more expensive to install, unless you are having the roof retiled anyway.
What is the ideal location for PV?
The efficiency of solar PV arrays is greatly affected by the amount of sunlight they receive. Optimum performance is achieved when the array is:
- facing between the southeast and southwest
- angled upwards at 30-45 degrees
- has little or no shading throughout the day, winter and summer
Crystalline silicon in-roof PV modules acting as slates
How much will it cost?
Costs vary a lot depending on the choice of PV module and size of the array; for example, solar tiles are more expensive than solar panels fitted above the roof. However, the total cost should lie in the range of £1500 to £2000 per kWp for a fully installed and commissioned grid-connected system.
Solar PV systems installed after 15 July 2009 and certified under the Microgeneration Certification Scheme are eligible to receive payments under the feed-in tariff (FITs) scheme, from the property's electricity supplier. For details of the FITs scheme click here.
Solar panels have no moving parts and require very little maintenance, apart from ensuring that they are kept free of debris. The rain generally keeps panels clean on a roof. Inspection of the inverter and other components should be carried out periodically by a qualified technician. The inverter is likely to need replacing during the operating life of the system.