Solar panels (photovoltaics) use sunlight to generate electricity. Photovoltaics can be roof-mounted, free-standing, or integrated into the building fabric.

Requires: A roof that is not north facing (in the UK or at similar latitudes), or suitable ground area.

Works well with: Hot water storage, Battery Storage, Electric Vehicle Charging Points, Heat Pumps.

Solar panels use photovoltaics to convert sunlight into electrical current. They produce electricity during daylight hours, even on overcast days, although they perform best in direct sunlight. As they rely on light, solar panels generate variable amounts of electricity over the course of the day and are quite seasonal. Larger systems are typically more cost-effective.

Photovoltaics can be:

• Installed onto roofs. Solar tiles and slates designed to blend in with the aesthetics of a building are also available. These are approximately twice as expensive but may be required for planning reasons. (1)
• Incorporated into roofs as building-integrated photovoltaics, serving the dual function of being a building skin and a power generator. (2)
• Installed on purpose-built auxiliary structures, for example over car parks.
• Free-standing on the ground. Some free-standing systems can track the sun, increasing generation by 20% or more, but this capability increases system cost and complexity. (3)

Photovoltaics that convert sunlight to electricity can be made of different materials. The very best panels are 22% efficient, although the most common ones are currently around 15%. (4)

Thick crystal technologies such as mono- or polycrystalline silicon wafers can generate 110-130Wm^-2 in full sunlight. Monocrystalline technologies are more expensive than polycrystalline but take up less space and are very marginally more efficient at higher air temperatures. (4,5)

Sunlit solar panels absorb energy, wasting some as heat. Solar panels are frequently much hotter than the air temperature. This decreases their efficiency. Efficiencies vary between products, but as a guide, polycrystalline technologies lose around 0.5% efficiency for every degree increase in air temperature. (6)

Thin film technologies consist of layers of very thin photovoltaically-active material on a glass or metal substrate. These can deliver 40-50Wm^-2 under full sun. Advantages of thin film systems are that they are cheaper and more flexible than thick crystal systems. (4)

Photovoltaic systems generate direct current, unlike the grid-supplied electricity which is alternating current. An inverter will need to be installed to convert generated electricity from direct current into alternating current. (1)

For greatest electrical output in the UK, panels should face south, tilted 30-40° from horizontal. In the UK and at similar latitudes, east or west-facing roofs still warrant consideration. North-facing roofs are currently unsuitable given present technologies and financial incentives. When siting panels, consider obstacles such as buildings, trees and chimneys that may provide shade thus reducing efficiency.

Where partial shading of a solar panel array may occur, lower cost installations will experience reduced effectiveness of all connected panels, unless higher quality inverter systems capable of controlling the current in individual panels are utilised in the system. This should be considered if the proposed location will experience transient shadows throughout the day.

Panels should last around 25 years and are generally low maintenance. The inverter may not last as long as the panels. Panels are cleaned by the rain if their tilt angle is greater than 15° from the horizontal. If dust, sand, or particulates settle on the panels they may need to be wiped, as their efficiency is reduced when dirty. For this reason, ground mounted panels will need cleaning more regularly than roof mounted panels. It is advisable to have panels serviced at least once every 3 years by a certified installer. (4)

Usually, photovoltaic installations are classed as permitted developments. (1)

It may be desirable to install a photovoltaic diverter so that, instead of being exported to the grid, generated electricity is used for heating a hot water cylinder or solar water heating, or assisting a whole-building heating system such as a heat pump. Alternatively, a battery storage system could be installed for the electricity to be used later.

(1) Energy Saving Trust, Generating Renewable Energy, Solar Panels. Available from: https://energysavingtrust.org.uk/advice/solar-panels/ [Accessed 26^th January 2021].

(2) S. Strong, Whole Building Design Guide, Building Integrated Photovoltaics (BIPV). https://www.wbdg.org/resources/building-integrated-photovoltaics-bipv [Accessed 26^th January 2021].

(3) R. Aldrich, Whole Building Design Guide, Alternative Energy. Available from: https://www.wbdg.org/resources/alternative-energy [Accessed 26^th January 2021].

(4) The Renewable Energy Hub UK, A Complete Guide to Solar Panels in 2020. Available from: https://www.renewableenergyhub.co.uk/main/solar-panels/ [Accessed 26^th January 2021].

(5) Enkonn Solar Energy, Polycrystalline vs Monocrystalline Solar Panel, Which One is Better? Available from: https://enkonn-solar.com/polycrystalline-vs-monocrystalline-solar-panel [Accessed 26^th January 2021].

(6) Quantum Photovoltaics Research Group, University of New South Wales, How Does Air Temperature Affect Photovoltaic Solar Panel Output? Available from: https://www.qpvgroup.org/blog/2019/2/3/how-does-air-temperature-affect-photovoltaic-solar-panel-output [Accessed 26^th January 2021].