Photovoltaics, or PV, is the technical name for solar electricity. It means free electricity from the sun (once you’ve invested in solar panels, of course). And it’s more common than you might think: garden lights, burglar alarms, even parking meters have small PV panels in them.

So how does it work? PV uses cells to convert sunlight into electricity – not to be confused with solar thermal, which is another type of panel (or rows of tubes) that uses the sun’s energy to heat your water. The PV cell consists of one or two layers of a semi-conducting material, usually silicon. When light shines on the cell, it creates an electric field, causing electricity to flow. The greater the intensity of the light, the greater the flow of electricity.

Panels are either used as an alternative to power from the National Grid, or to charge sets of batteries for homes not on the grid. PV cells are referred to in terms of the amount of energy they generate in full sunlight, known as kilowatt peak or kWp.

Installation

Ideally, solar panels should be south-facing and free of shade at an angle of between 20 and 50 degrees. panels are typically mounted on the roof, but can be on frames on the ground or even on walls. You shouldn’t need planning permission for them, but always check with your local authority: you will probably have to fill in a permitted development form. You could, theoretically, fit the panel to the roof yourself, but that would prevent you from claiming a grant of up to £2,500 under the government’s low Carbon Buildings programme (lCBp), which requires you to use one of its accredited installers. You will also need a qualified electrician to do all the necessary wiring. a possible exception is with solar tiles (see below).

New-build or existing home

This is a technology that you can easily fit to your existing home – they call it ‘retrofit’ in the building business. in the case of a new-build, you could consider making your roof out of photovoltaic tiles, rather than traditional tiles. They are more expensive than bolt-on units, but you will be saving on the cost of roofing materials. This is also one way you might be able to save on installation. it works like this. You can buy accredited products from established builders merchants, such as Travis perkins, then find an accredited roofer to fit the tiles, and an accredited electrician to commission the system. However, it’s a lot of project management for probably very little saving.

Standard panels can either be bolted on or sunk into the roof. a typical domestic installation is about 14sqm. There will be a tiny amount of disruption indoors with a little bit of new wiring, but we’re not talking about knocking down walls or pulling up the floor.

You will also need to find a home for an inverter, which converts the current from the solar panels into current that’s compatible with the home. This is a box about 25x25cm and usually goes in the loft space.

Innovative design

Julian Marsh is an architect with previous experience in green building. His company, Marsh Grochowski, was behind the high-profile renovation of the home of campaigning Labour MP Alan Simpson (see overleaf). Now, Marsh has set to work on his own Nottingham home (above and below). Among the innovations at the Meat Factory – its original function needs no explanation – is this PV canopy, bolted on to the outside of a ‘sun space’. It has a dual function, generating electricity throughout the year, and shade in summer. The canopy is angled to allow sunlight into the home in winter, meaning the living area benefits from passive solar heating, too. The 2.5kWp system cost £13,000 and sits in a specially made steel frame, which added another £3,000 to the job.

Tile option

This solar installation on a house in Somerset uses solar electric roof tiles. When the Merrill family decided to re-roof their home (below) in 2006, they chose to fit solar tiles on a section of the roof instead of a more traditional product. Solar tiles are more expensive than bolt-on panels (the Merrills paid £15,000 for a 2kWp system), but the family saved on the cost of normal tiles, while the scaffolding had to be paid for whatever they decided to fit. The performance of the 40 tiles, covering 15sqm of roof, has been better than they anticipated. The Merrills claim their electricity consumption is around one third less than the average UK household (using figures from the government- backed Energy Savings Trust). This is typical of families with micro renewables, who tend to be more aware of their energy consumption. And with the PV tiles, they say they have cut their bills by two thirds, down from £150 to £50 a quarter, an annual saving of £400.

Costs

A typical domestic system is 2 kilowatt peak (kWp). This means that the maximum possible generation is 2 kilowatts (kW). However, given the wobbly quality of British daylight and sunshine, any calculations you see are likely to be based on an average generation of 0.85 kW per kWp. Therefore, the average generation of a 2kWp system is 1.7kW.

The installed cost of a panel is around £14,000, including 5 per cent VAT. For tiles it is a bit more – say £15,000, for the same-size system. Value for money, however, is difficult to specify. Some PV systems are more efficient than others, some offer longer guarantees than others, and some are cheaper than others. Brands with a track record include Sharp, Sanyo and Sunpower.

Your supplier should be able to offer you a range of options, specifying the benefits of each one. If it specialises in one brand, you should compare this with quotations from other installers. Any decent system should be guaranteed for at least 20 years, simply because there is so little that can go wrong. And you can expect them to function for a long time after the guarantee runs out: early PV systems installed in Japan 40 years ago are still producing electricity today.

Grants

The main place to look is the Government’s Low Carbon Buildings Programme (lowcarbonbuildings.org.uk), where householders can get up to £2,500 towards the cost of a PV system. Depending on where you live, other grants might exist, so it’s always worth a call to your local authority.

Selling back to the grid

This is where PV gets really exciting, but first a bit of background. If you install PV solar panels, you are producing carbon- neutral electricity during daylight hours. However, because this is the time you are least likely to be consuming energy, excess power is exported to the National Grid, and you are paid for it. In the evening, when you need electricity for cooking, television, cups of tea and so on, you buy back electricity from the grid.

However, there’s a catch. Some suppliers offer measly rates for the electricity you export, often less than the rate they charge for what you buy. Scottish and Southern is one notable exception, offering 20p per unit of electricity, as long as you export at least half of what you generate. This is one reason why payback times are so difficult to calculate and vary wildly depending on who you talk to. Using the Scottish and Southern model, the installer Solarcentury has calculated that a household using an average amount of electricity, and with a typical- size PV system on the roof, could pay back its investment in 13 years.

From April this year (2010), however, it gets better. This will be when the Government introduces feed-in tariffs under the new Energy Act. These guarantee homeowners an above-market rate for the electricity they sell back to the grid, which should radically reduce payback times for PV.

However, the Government has yet to decide how it will work, and how much will be paid for each unit of electricity. The We Support Solar campaign has been set up by the UK Solar PV Manufacturers Association to make sure that the Government commits to fair and effective financial means to encourage investment in solar power. Show your support by signing up at wesupportsolar.net where you can also find more information about solar power and feed-in tariffs.

If you do decide to invest in a PV system, you will also receive renewable obligation certificates, or ROCs, for the electricity you produce. These can be exchanged for cash with the electricity regulator Ofgem. It won’t make you rich, but it’s certainly an extra perk. A 2kWp system should give you a return of £130-£160 a year.

Words: Dominic Murphy