Solar photovoltaics

Introduction

Solar photovoltaic (PV) systems use ultra-violet light from the sun to generate electricity. When installed on or near a building they can be used to run appliances or stored in a battery for later use, for lighting or to charge an electric vehicle, for example.

How solar PV works

A PV cell consists of a sandwich of a semi-conducting material, usually silicon, with layers of boron and phosphorous between layers of glass or flexible plastic. When light shines on the cell it causes electricity to flow around a circuit. The greater the intensity of the light, the greater the flow of electricity. 

PV systems generate no greenhouse gases in use so saving, in central Scotland, up to the equivalent of 400kg of carbon dioxide emissions per year for each kilowatt peak (kWp*) installed, which is around 5.5m2 of PV modules. If it takes approximately 5 years in Scotland to generate the equivalent of the energy it took to manufacture the system, a typical 2.5kWp domestic system will save around 20 tonnes over its 25-year lifetime.  

*kWp - PV arrays are rate in terms of the watts they can potentially generate under standard test conditions of irradiation and cell temperature, typically 250 to 300 watts per module (panel).

PV modules come in a variety of shapes and colours, ranging from grey 'solar tiles' that look like roof tiles to transparent panels that can be used, for example on conservatories, to provide shading as well as generating electricity.

Solar PV and your home

You can install a PV system on a building with a roof that faces within 90 degrees of South, as long as no other buildings or trees overshadow it. If even a small part of the system is in shadow for parts of the day, the output may decrease significantly. The ideal in Scotland is between SE and SW with a slope of around 36O to the horizontal, but it makes little difference if within the range of 25 to 45O. A steeper slope aids cleaning by rain.

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 the strongest wind without weakening the roof structure. A structural engineer may be approached for advice if there is doubt.

Solar PV installations should always be carried out by a trained and experienced MCS-accredited installer (see later); they are required to verify that a roof is suitable.

Costs and maintenance

Prices for PV systems vary with the size of the system to be installed, type of PV module used and the nature of the actual building on which the PV is mounted. Scaffolding can be a significant element in a building of more than a single storey. The size of the system is dictated by the amount of electricity required, the space available and of course the owner’s budget.  

For a domestic system, costs can range between £6-8,000 for a 5kWp system, to £2-4,000 for a 1.5kWp system, these being around 8m2 and 28m2, respectively, in size. Panels integrated into a roof will cost more than those that sit on top unless installed as the roof is being built. Solar tiles cost around twice conventional panels.

As installation costs can vary greatly between installers and products, we recommend getting quotes from at least two MCS-accredited installers. Click here for information on finding an installer.

PV systems require very little maintenance, generally limited to ensuring that the panels are kept relatively clean and that shade from trees and other vegetation has not become a problem. The wiring and components of the system should however be checked periodically by a qualified technician. Inverters and, if used, batteries may need replacing before the modules, possibly every 7 to 10 years. The cost of these items continues to fall.

The Benefits of Solar Electricity

Cut your electricity bills:   your electricity bills will be reduced. In Scotland expect a typical yield of roughly 800 to 900 kWh per year for each kilowatt peak of capacity. A typical 2 kWp domestic system will thus produce around 1,700 kWh, which is around half the annual electricity consumption of a typical household (not electrically heated), but how it benefits financially will depend on how it uses power during the middle of the summer days.

The majority of generation takes place between 10am and 4pm between March and October. Units exported are worth much less than those displacing imported units, perhaps only 3-5p compared to 15-18p. This is a reason to install a smaller system to increase the proportion of power used on site.

Store electricity:   for those who are away from home most days or have a larger PV array, or are not mains-connected, it is becoming increasingly attractive to install a battery system, typically with a storage capacity of 2 to 14 kWh, as these continue to fall in price. If you also use an electric vehicle (EV), charging it with stored power can make both even more cost-effective.

Cut your carbon footprint:   solar electricity is a green, renewable energy and doesn’t release any harmful greenhouse gases or other pollutants. A typical home PV system could save around 1,000kg of carbon dioxide per year – that’s around 25 tonnes over its lifetime.

Sell electricity back to the Grid:   if your system is producing more electricity than you need, or when you can’t use it, your neighbours will use it – and you could receive payment from your supplier via the Smart Export Guarantee (SEG).

For more detailed information about the Smart Export Guarantee, click here.

Planning considerations

Always check with your local authority about planning issues before you have a system installed. Most PV installations on roofs and some ground-mounted systems are Permitted Developments, meaning they do not need planning permission, but in conservation areas or on, or near, listed buildings, planning permission is required. Obtaining retrospective planning permission can be difficult and costly.