Solar photovoltaics

Introduction

Solar photovoltaic cells (PV) use ultra-violet light from the sun to generate electricity which can be used in a building to run appliances and lighting.

How solar PV works

A PV cell consists of a sandwich of a semi-conducting material, usually silicon, with layers of boron and phosphorous and a layer of glass or flexible plastic. When light shines on the cell it creates an electric field across the layers, causing electricity to flow. The greater the intensity of the light, the greater the flow of electricity. 

PV systems generate no greenhouse gases in use, saving approximately 375kg of carbon dioxide emissions per year for each kilowatt peak (kWp*) installed, around 6m2 of PV modules. If it takes up to 6 years in Scotland to generate the equivalent of the energy it took to manufacture the system, it will save around 5 tonnes over that 1kWp system's lifetime.

*kWp - PV cells are referred to in terms of the amount of energy they can generate under standard test conditions of irradiation and cell temperature.

PV arrays now come in a variety of shapes and colours, ranging from grey 'solar tiles' that look like roof tiles, to panels and transparent cells that you can use on conservatories and glass to provide shading as well as generating electricity. As well as enabling you to generate free electricity they can provide an interesting alternative to conventional roof tiles.

Solar PV and your home

You can use PV systems for a building with a roof or wall that faces within 90 degrees of south, as long as no other buildings or trees overshadow it. If the roof surface is in shadow for parts of the day, the output of the system decreases.

The roof must be strong enough to take the solar panels’ additional weight (around 15kg/m2) and the fixings must be substantial enough to withstand the effect of wind upon them without weakening the roof structure.

Solar PV installations should always be carried out by a trained and experienced installer.  

Cost and maintenance

Prices for PV systems vary, depending on the size of the system to be installed, type of PV cell used and the nature of the actual building on which the PV is mounted. The size of the system is dictated by the amount of electricity required.  

For the average domestic system, costs can be around £4,000- £9,000 per kWp installed, with most domestic systems usually between 1.5 and 2 kWp. Solar tiles cost more than conventional panels, and panels that are integrated into a roof are more expensive than those that sit on top.

In Scotland, expect a typical yield of roughly 850—900 kWh per year for each kilowatt of rated capacity. A typical 2 kW domestic system will thus produce around 1,700 kWh, which is half the annual electricity consumption of a typical household.

Grid connected systems require very little maintenance, generally limited to ensuring that the panels are kept relatively clean and that shade from trees has not become a problem. The wiring and components of the system should however be checked regularly by a qualified technician.

Stand-alone systems, i.e. those not connected to the grid, need maintenance on other system components, such as batteries.  

THE BENEFITS OF SOLAR ELECTRICITY

Cut your carbon footprint:   solar electricity is a green, renewable energy and doesn’t release any harmful carbon dioxide or other pollutants. A typical home PV system could save around 1200 kg of carbon dioxide per year – that’s around 30 tonnes over its lifetime.

Cut your electricity bills:   sunlight is free, so once you’ve paid for the initial installation your electricity costs will be greatly reduced. A typical home PV system can produce around 40% of the electricity a household uses in a year.

Sell electricity back to the Grid:   if your system is producing more electricity than you need, or when you can’t use it, someone else can use it – and you could make a bit of money.

Store electricity:   if your home isn’t connected to the national grid, you can store excess electricity in batteries to use when you need it.

Planning considerations

Some local authorities require planning permission to allow you to fit a PV system, especially in conservation areas or on listed buildings. Always check with your local authority about planning issues before you have a system installed. Obtaining retrospective planning permission can be difficult and costly. 

The average domestic solar PV system is 3.5kWp and costs around £7,600 (including VAT at 5%).

Costs have fallen significantly over the last year. They vary between installers and products, so we recommend getting quotes from at least three installers. Guidance on finding an installer.

Other factors that affect PV installation costs are:

  • The more electricity the system can generate, the more it costs but the more it could save.
  • Larger systems are usually more cost-effective than smaller systems (up to 4kWp).
  • PV panels are all around the same price per kWp, but PV tiles cost much more than a typical system made up of panels.

Panels built into a roof are more expensive than those that sit on top