Solar Energy

Topic 4: Solar Energy


There are two different types of renewable energy both referred to as ‘solar panels’. One type is solar thermal panels, which collect heat from sunlight and transfer this to water systems. The second is solar photovoltaic panels which generate electricity from sunlight. This module will mainly concentrate on solar PV systems with some overview information covering Solar Thermal.

Before looking more closely at PV it is worth comparing the two systems to help you understand the pros and cons of each.

Solar PV

Here is a solar PV system. It is an array of panels. Each panel is made up of modules.

Solar Thermal

Here are solar thermal collectors. From a distance they look similar to solar PV panel. These are flat plate collectors. Other types are evacuated tubes, which look like a row of glass tubes placed very close together.

System Requirements

Solar thermal systems require collectors on the roof (the panels), a fluid to collect and transfer the heat, a pump to pump the fluid between the panels and hot water tank, installation of pipe work, expansion vessel and usually an oversized hot water tank in order to maximize the efficiency of the system. The image below shows a tank, expansion vessel and pipes bringing in heat from the solar collectors.


Solar PV requires the solar panels on the roof, an inverter to change the output of the panel from DC to AC, upgraded wiring and a method to measure generation. A display showing the cumulative and current generation is optional but popular. The image below shows an inverter on the side of a building.


Solar thermal systems have more mechanical elements and therefore require more maintenance. The pump, collectors, fluid and expansion vessels can all require attention to check they are correctly operating and not losing efficiency. The pipework should be insulated and the hot water tank correctly sized and any back up immersion heaters timed correctly to match demand and generation.

Solar PV requires less maintenance. In theory once installed you should not have to interact with the system other than to check generation totals, submit any FIT claims, cut back any tree growth which might be oversharing, and replace the invertor at the end of its lifespan. However it is recommended to ensure your panels are kept clean to maximise efficiency, if it hasn’t rained for a while consider rinsing your panels to get rid of dust and debris.

How Does it Work?

Solar thermal panels absorb heat from sunlight and transfer it to a tank via a heat transfer fluid. the tank is usually topped up by either an immersion heater or a boiler.


Solar Photo Voltaic panels (to give them their proper name), are very different. Solar PV panels work by using the sunlight to generate a flow of electricity. The stronger the sunlight the more electricity they can generate. This ‘strength’ is measured by something called ‘solar irradiance’. If you want to understand more about how PV works then NASA has the answers.

Solar PV technology first took off in the 1970’s as a response to the oil crisis. Since then the technology has steadily been progressing by improving efficiency and reducing costs. Average efficiency is now around 20%. Although as the energy input (sunlight) is free, efficiency is less important than in say an old oil boiler which might have an efficiency of 70%. Which means that 30% of the oil you buy is wasted.

Different types of panels have different typical efficiencies. Mono-crystalline is more efficient than poly-crystalline, but also more expensive. Thin-film solar panels are the cheapest but lowest efficiency type of panel, typically only 5-10%.

If you are interested in sustainable energy then a fantastic freely available book is ‘Sustainable Energy Without the Hot Air’ by Sir David MacKay.

East or West Facing roof?

The direction your panels face will determine the total amount of energy generated but also at what time of day the majority of the energy is generated.

If you have a south facing roof then panels should be installed on the south side.

But what if your roof is east/west facing? Panels installed on the east side of a roof will generate more electricity in the morning, and panels installed on a west facing roof will generate more electricity in the evening.


  1. Find out the orientation of your roof using a compass or map
  2. Look for any sources of shading that will affect generation
  3. Determine the points of the day when the majority of the light hits each side of your roof. This website provides a handy way to track the path of the sun throughout the year at any location. Just type in your location and it will show sunrise, sun set and the path of the sun in between. You will still need to identify any significant sources of shading for your particular roof.
  4. Or you can also do it more simply by using your own knowledge of the suns path across your site during the summer months.

Now you know when the majority of the electricity will be generated, next it’s time to work out when you use the most electricity.

  1. Get your OWL,
  2. If it’s a WIFI enabled OWL then log into your account using your tablet or computer
  3. If it’s a USB OWL then download the data using the instructions provided in the user guides. If you have a Micro+ you will not be able to access historic electricity consumption by time of day unfortunately.
  4. Is the consumption flat throughout the day? Or is there a peak in either the morning or evening? Once you know when you use more electricity you can decide which side of the roof would best suit your individual energy consumption profile.

Communicating with Guests

Pointers and tips for:

Guests in self-catering accommodation – If you decide to install solar PV then it is a great idea to share this fact with your guests.

It is possible to also install a display panel which logs how much electricity you have generated in total and what is currently being generated. You may be able to automatically feed this into your website in order to show potential guests your EcoTourism credentials.

Below we have put some suggested text to personalise and include in welcome packs for self-catering accommodation. You will need to check and amend it to ensure it’s correct for your particular site. Bear in mind that some research shows that guests are not necessarily as conscious of energy and environmental issues when they are on holiday as when they are at home, so encourage and enable them to help you and the Islands reduce carbon emissions.

Dear Guest,

As part of the Smart Energy Islands project, the community of the Isles of Scilly is working together to reduce energy consumption and improve the sustainability of the Isles of Scilly. We are generating our own electricity via solar panels on our roof and using this to provide you with zero carbon hot water. Please help us by being mindful of energy use during your stay. To help you we have compiled a few tips you may choose to follow:

  • Put lids on pans while cooking
  • Turn off lights when you go to bed or go out
  • Turn off towel heaters during the day

Customers in food and drink businesses – If you have PV installed at your pub, café or restaurant, you too can install a display panel to tell all your customers that you are generating renewable energy.

You may wish to also add information to your website informing customers that their visit to you will be solar powered!

Further Information

The British Solar Photovoltaic Association has web pages on the history and benefits of solar PV.

For more detailed technical information especially on the interaction with battery storage, the Solar trade Association has some interesting reports to read:

The NASA science pages also have a great explanation of the science behind solar cells.