Solar panels are one of the most recognisable forms of renewable energy sources, but you’d be forgiven for thinking that they’re not suited to maximising their full potential in the UK.
However, while spells of sunshine may be short lived in this part of the world, there’s still a significant amount of energy that can be harnessed from the sun via solar panels.
Before you disregard this energy source entirely, it’s important to arm yourself with the right data. Localised information is available from websites such as PVGIS (Photovoltaic Geographical Information System), which gives up-to-date data on solar irradiance in different areas.
However, when it comes to solar panels, it’s not one size fits all. There are different types best suited for different purposes – your choice of which will affect the cost and functionality, as well as the aesthetics of your home and which grants are available to you.
What Are the Main Types of Solar Panel?
There are two types of solar panels:
The first type of solar panels are photovoltaic (PV), which generate electricity from sunlight. PV panels measure around 1600 x 1000mm.
The second type are solar thermal panels, which generate heat for use in your domestic hot water cylinder.
There are two main types of solar thermal panels – flat panels that measure around 1000 x 2000mm and evacuated tubes that will have around 20 glass tubes that are made into a single collector.
Photovoltaic thermal panels, or solar PVT, are a combination of solar photovoltaic panel and solar thermal panels in one module. They are sometimes known as hybrid solar panels.
How Do Solar Panels Work?
Solar PV panels have very thin silicone cells that are doped in chemicals to make them sensitive to light. The light causes a reaction between the cells and an electric current is produced. The brighter the sunlight the greater the energy generation.
Solar thermal panels have a dark coated metal sheet in them that has copper pipe attached to it with water running through it. There is insulation behind the pipes and therefore any heat that is absorbed by the dark panel is then reabsorbed by the fluid in the pipe and moved to a hot water cylinder.
How Many Solar Panels Are Needed to Run a House?
Solar PV panels are only around 20% efficient, so you would expect to see between 12 to 16 panels on a typical 4kW system.
Solar thermal panels are actually around 80% efficient so you would expect to fit one flat panel (or 10 tubes) for every 100 to 120 litres of hot water storage capacity. Therefore if you have a 240 litre cylinder you would have two panels but a 300 litre cylinder may require three panels or 30 tubes.
While the effectiveness of solar panels is weather dependent, there are ways to optimise their efficiency.
The best fixed position for a solar panel in the UK is to face south on a pitch of around 35°. Any deviation from this will have a negative impact on the annual energy yield. The roof must be structurally sound enough to take not only the weight of the panels, but also the wind lift that may be caused by wind getting behind them.
With PV panels, it is crucial to avoid any shading – even where one panel is in the shade, such as behind a chimney stack. This could cause the whole array to stop generating energy.
If shading is a risk then special design considerations need to be applied to mitigate or limit the reduction in generation.
How Much Do Solar Panels Cost?
On a typical 4kWp on-roof solar PV array we would expect to see between 12 and 16 panels (depending on the output of the individual panel) and the fully installed cost would be somewhere between £5,000 to £8,000 depending on the quality of the panels and the complexity of the site.
If the panels are integrated into the roof or are made to mimic the tiles or slates then the fully installed costs of a 4kW system would be expected to be around £10,000 to £15,000.
Solar thermal systems will usually require a new cylinder as you need two coils in it, one for the solar panel circulation and the second for the back up heater such as a boiler (gas, oil or biomass), or a heat pump.
Depending on the roof access and the cylinder location the cost to install a solar thermal system could be between £5,000 and £8,000.
Are There Grants Available for Solar Panels?
Solar PV used to have the benefit of a very generous incentive payment through the Feed In Tariff (FiT) scheme but that has now ended.
There are, however, buy back rates available from the energy suppliers where they will pay you for your excess generation. This payment is not fixed or prescribed so it is best to shop around for the best deal.
Solar thermal panels currently benefit from incentive payments under the Renewable Heat Incentive but this scheme is due to end in March 2022 and we are currently unsure what it will be replaced with, if anything at all. The proposal is to create a one-off payment scheme under what is being called the Clean Heat Grant; however, this is currently under consultation.
Under the Green Homes Grant, solar thermal panels are included as a primary measure, but PV panels have specifically been excluded. This scheme is currently planned to operate until March 2022 also.
Do I Need Planning Permission for Solar Panels?
In most cases, solar panels will often fall under permitted development rules. These rules include amongst other details that the panels must not protrude more than 150mm off the profile of the roof, must not be higher than the highest part of the roof (not including the chimney) and must not face the road.
In all other situations then advice should be sought from the local authority before proceeding. Listed buildings and properties that fall within conservation areas or Areas of Outstanding Natural Beauty (AONB) will also have separate rules when it comes to the planning permission to fit solar panels.
How Are Solar Panels Installed?
Solar panels are installed either on the roof above the roof cladding, integrated into the roof either mimicking the cladding or as panels that are flush with the cladding.
Roof hooks are fixed to the roof trusses or rafters and aluminium rails are then fixed horizontally to the hooks. The panels are then bolted to the rails and (with PV) all the wires fed back to a device known as an inverter, which converts the generated energy into useable electricity for use in the home.
Solar thermal panels are also bolted to roof hooks but will have pipes connecting to a cylinder in the home.
Are Solar Panels Worth it?
In the age of the FiT, PV panels were an exceptionally good investment. Not only did you get paid for the generation but you could use it too, therefore saving on buying in the electricity.
Now that the FiT has ended it needs to be calculated how much energy you will generate annually, what percentage of that energy will be used in the home, what the cost is of energy that is being displaced by the PV generation and therefore what the potential payback would be.
For example if you have natural gas going to your house and the cost of electricity is 15p/kWh and the cost of gas is 5p/kWh then if you use your generation to power your fridge and your washing machine you would offset 15p per kWh used. If, however, you redirect your generation to the hot water cylinder then you are actually only saving 5p per kWh.
David is an expert in sustainable building and energy efficiency and is also director of Heat and Energy Ltd.
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