Photovoltaic technology on a small scale is probably familiar to most people in the UK. It is used to power calculators, road signs, toys and phone chargers. Taking light from the sun, it uses it to run the appliance. Photovoltaic panels for electricity work on the same principle, just at a larger scale.

Sunlight is all you need to get electricity from the panels. It is an unlimited resource that’s never going to stop shining (even though it doesn’t always feel like that in the UK). Although more electricity is produced on sunny days they work well on overcast days too.

There are lots of benefits of a PV system. It is easy to install, needs virtually no maintenance and is estimated to last 40 years. Research at the EC Joint Research Centre has found that most of the panels they have tested continue to provide more than 92 per cent of the initial power after 20 years. They are suitable for use in urban areas which wind or hydro systems don’t tend to be.

The main disadvantage is the cost which is currently higher than for other renewable energy sources (if you don’t take the environmental costs into consideration). Some countries in Europe, such as Germany and Spain, have feed-in tariffs which pay a premium rate for electricity generated (or in some cases just the amount exported). This makes the cost of installing a system much more attractive. A feed-in tariff (Clean Energy Cashback) will start in the UK in April 2010. Anyone who installs solar panels after 15 July 2008 will be transferred onto it automatically. The rate for installations up to 4kWp is 41.3p per unit generated, plus 3p per unit exported, plus the savings on your electricity bills. Click on the link for full details of feed-in tariff rates.

• Photovoltaic systems use cells, consisting of one or two layers of semi-conducting material, to convert solar radiation into electricity. The semi-conducting material is generally silicon, which is the second most abundant element on earth after oxygen.
• light shines on the cell creating an electric field across the layers
• this causes electrons to flow creating electricity
• on its own each cell only creates a tiny bit of electricity, but connected together to form panels, which are linked together to form a system, they create useful amounts
• panels are mounted on the roof or on a frame
• an inverter converts the direct current (DC) to alternating current (AC – or mains equivalent) electricity which is suitable for running appliances
• grid connected systems can export electricity they don’t use to the grid, and import it from the grid when there is not enough sunlight
• off-grid systems store excess electricity in a bank of batteries
• off-grid systems can be used in conjunction with other sources of power such as biomass boilers, wind or hydro turbines.

Ideally a photovoltaic system should face between south east and south west, and be free of shade. For best performance they should be angled at 30 to 40 degrees – although you will still catch a reasonable level of sunlight at angles of 20 – 50 degrees.

Click here for a tool that calculates how much potential your site has to generate electricity from PV.

Solar panels weigh quite a bit, so your roof must be strong enough to hold them. If you need to re-roof, you can do so using solar tiles. These are more expensive than panel systems, but if you are re-roofing anyway, it can be more cost-effective to re-roof with solar tiles than to use conventional tiles and put panels on top.

The photovoltaic industry is developing fast, so it’s worth asking a few suppliers what they recommend.

Crystalline silicon technology is the most commonly used in the UK and the most efficient at converting sunlight into electricity. It consists of thin slices of silicon cut from a single crystal (monocrystalline) or from a block of crystals (polycrystalline). However, it is expensive to manufacture, so it costs more.

Monocrystalline performs most efficiently with modules typically converting 15 per cent of solar radiation into electrical energy, and with the best products reaching 21.5 per cent. The higher price reflects this. Polycrystalline silicon converts between 8 and 12 per cent of solar radiation into electricity.

Thin film technology involves depositing very thin layers of photosensitive materials onto a low-cost backing, such as glass, stainless steel or plastic. This technology is cheaper and more suitable for mass production. However, the efficiency rates are correspondingly lower. There is a range of types available:
Amorphous silicon (4-6 per cent efficient)
Cadmium telluride (7 per cent efficient)
Copper indium diselenide (9 per cent)

All types of PV system are measured according to their peak power rating which is measured in kWp (kilowatt peak). This is a guide to how much power the module produces under standard test conditions: it measures the power produced under 1kW per m2 of light. The more efficient the module, the smaller the array needed.

How close to peak power a PV module performs will depend on the intensity of light shining on it. It also varies between makes of module. The PV-Compare project run by the University of Oxford’s Environmental Change Institute found that the number of kWh of electricity produced by a 1kWp PV array varies significantly, with the highest yielding modules producing nearly twice as much energy as the lowest yielding ones.

The PV-Compare results (published in 2003) found that, in the UK, copper indium diselenide and double-junction amorphous silicon produce the most energy per Wp. Monocrystalline silicon and copper indium diselenide gave the highest yields of energy per unit area, making them most suitable for people with limited space.

Size of system will depend on which type of PV cell you choose, how good your site is, which part of the country you live in and how much electricity you want to generate. As a rule of thumb, a 1kWp system will generate an average of 850kWh of power in the UK. Most domestic systems are between 1.5 and 3 kWp.

The cost of photovoltaic systems tend to range between £5,000 and £8,000 per kWp installed. The cost of producing PV systems is falling and is expected to continue to do so.

The cost of PV equipment doesn't vary much as there are only a few wholesalers in the UK, and variability in installation costs and profit margins tends to be smaller than for other technologies according to research by Oxford University's Environmental Change Institute. 
Fixed costs £2,000
Equipment £5,000 per kWp installed
Example:  1kWp installation should therefore cost around £7,000, and a 2kWp installation around £12,000 (although there are signs of a 10% price drop over the two years analysed, and prices have since reduced further).

From April 2010, the Clean Energy Cashback (CEC - the government's financial incentive scheme) will guarantee an income for electricity generated at 41.3p per kWh generated, plus 3p per kWh for each unit exported to the grid for systems smaller than 4 kWp; 36.1p for 4-10kW systems; 31.4p for 10-100kW; 29.3p for 100kW-5MW and for stand alone systems. The CEC payments will be index-linked, and will continue for a 20 year period. To qualify for this feed-in tariff you must use a MCS accredited installer and product.

To find out if there are any local grants available in your area you can ring your local energy efficiency advice centre on 0800 512 012.

Solar panels are generally considered ‘permitted development’ in England and Wales as long as they are not installed above the ridge line of your home. However, if you live in a listed building or in a conservation area, restrictions will apply. Up-to-date advice is available on the government’s planning portal.

For those of you living in Wales, from the beginning of September 2009, the Welsh Assembly Government announced new planning rules to encourage householders to install renewable energy equipment. A leaflet has been published to explain the changes - Domestic microgeneration permitted development: A guide for householders.

From the Blog

• Efficiency or cost per watt - which is most important?
• Grants for Solar PV projects on community buildings
• Optimising solar PV siting
  
• PV systems need an efficient inverter
• PV shipments grow as community grants dry up
• Solar panels are flexible and easy
  
• Support for microgenerators increased this month
• Is my roof too shaded for solar PV panels?
• How to deal with snow on solar PV modules
• What's the best mounting for solar panels?
  

Sources:

• Centre for Alternative Technology
• Energy Saving Trust
• European Photovoltaic Industry Association
• Photovoltaics in the UK: An introductory guide for new consumers, Environmental Change Institute, University of Oxford
• The Guardian
• Friends of the Earth