PV sited where you want it

This method of using the sun's energy is solely directed at producing electricity. Although restricted to daytime only, modern systems can make use of the light striking the panels on days with high cloud and no visible sun. Photovoltaic systems are normally connected into the Grid, although it would be possible to store the extra electricity produced for use at night etc.

To find recommended suppliers/installers in any part of the world, manufacturers are now starting to place lists on their websites. This means that by contacting a recommended installer, you can be assured that the installation will be of the standard anticipated by the guarantee. This is especially important for PV systems where the choice of Inverter can be crucial to maximum generation on your site.

Please mention TigerGreen whenever you contact any of our linked Top Providers, Suppliers and Contractors.

We can help you to decide on the best combination of Renewable Energy Technologies for your home or business.

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How might it work for you

As the picture above shows, your panels can be sited where and how you want them. Solar Aid works in Africa to provide light inside houses so that people can study after dark, when their day time work is finished. Wisely, they haves placed the panels on stands on the roof. This allows good circulation of air to cool the panels during the day. If laid only a few centimetres above the roofing tiles or slates as we would do in UK, then the corrigated iron would cause the panel temperature to soar above its optimal maximum of 25°C. The use of a stand also means that the angle of each panel can be altered to track the sun from East to West each day, thus keeping the light at the optimal angles to the sun. You can learn more about Solar Aid by following the link on the right-hand side.

Normally Solar Panels are either attached to your roof through the tiles as shown in the photograph; or are actually set into the roof level with the tiles/slates, and the whole is made weatherproof. However, a stand-alone array can be set up on the ground and even set to track the sun. In the tropics this is done very simply by turning a handle.

You will need a south facing roof slope or area. Currently installers are working with slopes of approx. 35° - 37° to the horizontal, that are not overshadowed by trees, buildings etc. Think specifically of the situation in the winter when, in most parts of the UK you will be lucky to get a full 180° of sun at an high enough elevation to cause even small amounts of electrical generation. It is very important that the panels can intercept as much of the light as possible. A report by Prof. Anne Jacobson of Humboldt State University states that the best results are obtained if the panels at latitudes above 20°N are tilted at 35° to the south, and those above 40°N are tilted at 55° to the south. In the UK we are between latitudes 50 and 60° so the optimal angle of tilt will be over 55°to the south. Bear in mind that:

  • the best returns will be got by having the panels at right angles to the sun's rays.
  • you will require maximum electricity in the winter (Cooking, Lighting and Heating).
  • the elevation of the sun in the winter is low, the maximum height attained being dependent on latitude. So, in UK, the winter angle should be steeper than the degree of latitude.
  • on a cold winter's day sunrise is usually followed by cloud and it is only when this 'burns off' that the sun really comes through. Therefore, SW facing panels will always do better than SE facing ones and south facing is always the winner.

If the aspect and slope is right, then measure the total area of your roof, to give an idea of how many panels you could fit over it. Remember that panels need to be set in from the edge of the roof by 30cm to avoid being ripped off by high winds. Having got the area available, next check the size of the make of panel that you want - they are all much the same size but do differ slightly.

Another consideration concerning the siting of your panels, is that airflow is important. The power output of a PV panel decreases by 0.5% for every increase of 1°C.

Next you will need to know how much electricity you have used in the past year (total kWh). This gives you a rough idea of how much you need to generate to cover a reasonable percentage of your annual usage - preferably 100% plus some extra for the Grid.

The panels supplied in the UK are likely to be 2m long by 1.5m wide and the depth is <10cms; but they could be only 4ft X 2ft depending on where they are made. You will now be able to state the rough max. number of panels that you would need.

The next stage is to get an idea what other methods of electricity or heat generation would be appropriate for your property. Bear in mind that extra electricity can be sold back to the grid and will attract grant in the form of a FiT (Feed-in Tariff). The more you produce, the more money you get, and the less fossil fuels are used to supply electricity to the populace as a whole.

When you finally speak to an installer, they will tell you what each panel that they supply can produce - the kWp (kilo-Watt peak) of the specific type of panel. This rated value is the maximum production in full sunlight.

The Cost is high at £2,000 - £2,500 for the installation of a 1kW rated system. However, there are economies of scale and it is worth remembering that that 1kW could be produced for each of the 12 hours of full light on a summer's day and several hours of a winter's day. See 2 below for examples.

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Facts - Factors Affecting Energy Produced and Case History in UK

The solar radiation hitting the earth's atmosphere is 1.360kW/m² (New Scientist 24/11/2007 P45), of this a lot gets absorbed, reflected by the upper atmosphere and the clouds. However, each year the surface of the earth receives an average of 1068kW/m²/yr, and the UK gets an average of 150W/m² on the soil surface. This anticipates a surface that is 100% absorptive - panels are not! The panels vary and are improving but current efficiencies tend to be between 8% and 13% - some state up to 30%. The range in efficiencies, in optimally sited panels, is due to: the range of different types of Solar Panel being produced; and the temperature of the surrounding air.

Researchers in USA, Japan and the UK have shown that at temperatures of 40°C circa 13% of the energy produced by a solar panel is lost. To work get the best performance out of photovoltaic panels, they must be kept as cool as possible. This means that, globally, whereas the tropics may be ideal for solar furnace arrays, it is sites in the high latitudes and the tops of snow clad mountains in the lower latitudes that will provide the highest returns for solar PV. Location of PV in these areas will not only help some of our poorest and smallest communities; but HVDC below ground cabling could bring the solar electricity safely to areas of high population density and feed in points to the AC grids.

The idea of efficiency also seems to differ with the reporter, in that Tier 2 (the newest technologies) is supposed to be much better and more efficient than Tier 1, yet the Efficiency figures do not seem to bear this out.

Tier 1 rigid Solar PV Technology:-

  • Mono Crystalline Silicon is the most costly requiring 1,500°C to produce the panels but with an efficiency that some rate as high as 22%.
  • Poly Crystalline Silicon made of short lengths of Mono Crystalline, packed into a layer. This is cheaper to make and is allegedly 13 - 18% efficient.
  • Hybrids of the above - using a thin layer of one over the other are supposed to be 30%+ efficient.
  • Thin film Silicons and Cadmium-Telluride films. The former are approx. 8% efficient; and the latter, first made by First solar in 2009, are said to be 11.5% efficient.

Tier 2 rigid solar PV Technology - here they are going for the cheapness to produce a panel and its efficiency:-

  • CIS (Copper-Indium-Sulphide) panels and
  • CIGS (Copper-Indium-Gallium-Selenide) panels are 13+% efficient, and are being manufactured in 2 feet and 3 feet X 4 feet panels. A 900MW facility is being set up by a Japanese factory and more are being manufactured in Silicon Valley.

Case Histories can be found across the web, but as a guidance: a 3kWp (kilo-Watt peak) system in the south-east of England, comprised four 1.5m X 2m panels. This produced 2.730MW in its first year and 2.703MW in its second year, thus cutting about £400/yr off its owner's electricity bill. It would have paid for mine completely plus an additional 1.1MW to gain FiTs and be sold to the Grid!

Another report came from a retired couple in North Yorkshire, who installed panels on the southern side of their roof ridge. They have been saving their complete bill and been selling to the Grid every year since.

The initial costs of PV being very high, the FiTs (Feed in Tariffs) are also quite high. They are all payable for 25 years and work out at 36.1p/kWh on a new build and 43.3p/kWh on a retrofit at up to 4kW rating for the whole system. Installations of over 100kW - 5MW and Stand-alone systems are paid at 29.3p/kWh. The lower rate for the most powerful systems reflects the fact that these are producing electricity on a commercial scale and as such their operation is a business in its own right and not simply an asset to help run a farm business or a home.

The electricity you produce will also be paid for by the importing company at a current Export Tariff rate of 3.1p/kWh. If you are a charity or community then there may also be specific grants available to help you start-up.

This is an up and coming source of Renewable Energy with Germany on line to have 8GWp installed by the end of 2010.

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Pros and Cons


  • In a fully insulated, double glazed house, with a south facing unshaded roof, this technology attracts a grant to help with the initial outlay and connection charges where they are applicable.
  • It is low maintenance and robust.
  • It is less expensive initially than a Wind Turbine.
  • Frequently, it does not present a problem for Planning Permission.
  • It attracts FiTs at 36.1 to 43.3p/kWh for up to 4kW systems and slightly less for the more powerful systems.


  • It is more expensive initially than Ground Source or Biomass systems.
  • An annual wash down might prove worthwhile; certainly it is worthwhile brushing off the snow!
  • Planners may not allow it on some town houses if visible to the public.
  • Probably, it will not be allowed on a listed house. However forward thinkers may recognise that the result will be that: the owners will then be able to afford to maintain the house better.
  • Free standing units can be set up to track the sun from East to West. However, this daily tracking system has not proved robust in Spain, and it is best simply to leave them facing south and simply adjust the angle for high summer sun (30° - 45°) and low winter sun (45° - 65°) in the UK.
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Variations on the theme

PV (photovoltaic) arrays can be used for heating from the electricity that they produce, though this is probably less efficient than the other methods of producing heat directly.

Perhaps the best use of your south facing roof space would be to mix both PV and Passive solar heating panels.

Some of the biggest solar arrays use mirrors that follow the sun and direct its rays onto a water chamber. The steam produced drives a Solar Steam Turbine.

Many of the chemicals being researched now are applicable to curved surfaces and even to glass windows, where they are thin enough not to interfere with the passage of light to the interior. Still other applications are concerned with the application of films to flexible materials e.g; Rucksacks, sledge covers, parkas, and tent materials to name but a few.

Research is continuing at an ever increasing speed to find better and more efficient chemical combinations to use in the panels, so watch the news.

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How and Why it works

Solar Energy makes this type of electricity generation a viable possibility in most areas of the world. By the time that the Solar Radiation reaches Earth, by Infrared Radiation (Electromagnetic radiation of wavelength 780nm - 1mm), a lot of the energy has been lost and only approximately 2.7 x 1024J (½ a billionth of the initial energy) reaches the upper atmosphere. Travel through the atmosphere reflects 65% of this remaining energy; with the result that the ground receives an average of 0.8kW/sq.m and a max. of 1.2kW/sq.m. It is this remaining energy that is available to Solar Panels. Therefore, if the panels were composed of a substance that could absorb 100% of the incipient energy, then each square metre near the tropics would produce 0.8kWh of electricity for the home. Unfortunately, such a perfect substance is not available as yet, and therefore further losses ensue.

The panels work because of the nature of the films of material inside them. When light or other forms of energy strike an atom, they disrupt their electrons (negatively charged), which normally whizz around them at different distances from the nucleus (positively charged). The distance of the electron from the nucleus depends on the amount of energy it has. If the incoming energy is sufficient to knock the electron out of the pulling power of the nucleus, then the atom as a whole becomes positive and so little currents are set up causing a current to flow in the attached wires.

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Top Tips new and used

  • PV Panels lose out on dull days or facing north!

    The difference between achieving the maximum output from the photovoltaic panels on your house roof and the minimum possible on a day of low cloud cover is c. 15kWh for a normal 3bb house. A contact recently told me that he had been shocked to find that in dull daylight, or on a North facing roof, the min. was a mere 1kWh, whereas with direct sunshine onto the panels, the output was a healthy 16kWh. It is essential to site this expensive equipment on the south side of your house to gain the max. electrical generation. NB sunrise usually produces a haze that can last until mid-morning; whereas the afternoons are often clear. Therefore, south to south-west is best!

  • There is now an ever greater need to be aware that whereas PV panels will work in any light, they will only be able to maximize their output if unobstructed and facing south. Don't be beguiled by keen salesmen into putting these expensive electrical generators in sub-optimal positions. In all circumstances the best payback will be achieved by using the electricity that you have generated; so ensure that applianaces are used in daylight hours where possible and research the best ways of storing the energy that you produce, rather than feeding it straight into the Grid. Bear in mind that your home generation is saving Grid electricity for those users, who for one reason or another cannot produce their own - there is no reason for them to make accusations of 'using the system'!
  • PV 'payback time' surprises - yet another reason to never sign up for an installation without first checking the manufacturer's website no matter what assurances or bargains the installers offer you. When most inquirers ask about the 'payback time' they are thinking of the number of years that will pass before their purchase generates enough electricity to repay their investment of say £20,000. This is normally 6 - 10 years. Yet recently, 2 salesmen for photovoltaic panels assured me that the payback time for their REC panels was just one year, because they were so efficient. This seemed too good to be true, but they were adamant, so I decided to check the facts. A visit to the Renewable Energy Corporation site (REC) - Norwegian company who manufacture their own silicon etc. and used biomass for their energy needs in Norway - revealed that their panels take approx. 1 year to payback the amount of energy used in their construction. A respectable payback time for manufacturing energy; but alas, not 1 year to repay the cost price.
  • Another tip for those considering solar energy. Be it photovoltaic panels or any other agreements: read the small print very carefully. Door to door salesmen have offered me 3kW of free electricity for the 3kW panels they wanted to put on my roof! I don't think that they were actually trying to 'diddle' me; they just did not understand what was meant by a panel rating of 3kWh. I had to explain that panels were rated at 25°C and an irradiance of 1kW/sq.m, and in these ideal conditions those panels could theoretically produce 3kW every hour. However, in reality the output of a panel is also reduced: according to the time of day; the temperature and the rated % tolerance of the panel; the operating voltage; etc. In reality one could get 75% of the stated output on a tracking panel; but a fixed panel is unlikely to give >60% of the manufacturer's rating over the years. Calculate on this basis and you may be pleasantly surprised. This still makes a worthwhile investment, considering that panels are known to last 55yrs and counting!
  • Another tip for those considering solar energy. Be it photovoltaic panels or any other agreements: read the small print very carefully, on any agreement with installers who offer to rent your roof space. Door to door salesmen to-day offered me 3kW of free electricity for the 3kW panels they wanted to put on my roof! I don't think that they were actually trying to 'diddle' me; they just did not understand that it was the rating of the panels that was 3kWh (peak). I had to explain that, at 25°C and an irradiance of 1kW/sq.m, those panels could produce 3kW every hour. However, in reality their output is reduced: according to the time of day; the temperature and the rated % tolerance of the panel; the operating voltage; etc. In reality one could get 75% of the stated output on a tracking panel; but a fixed panel is unlikely to give >60% of the manufacturer's rating. Therefore, the amount of electricity generated should be 3kW x hours of direct sunlight, plus the lower amount of electricity produced under conditions of lower lighting e.g. if one gets 4h sunlight over 150 days, the panels would produce 3 x 4 x 150 = 1.8MW (1,800kW); to this one could add an estimate of the hours of all the other days of the year on which there was some good quality light - even if not direct sun. So, with luck one should get >2MW.
  • Photovoltaic panels are excellent and work very well in the UK, as they require light rather than heat, and we have some of the clearest skies around. However, if you have a property within 1 mile of the coast, then be aware that this may invalidate the warranty on the panel. It appears that the weak point is related to the interaction of salt in the atmosphere and the plastic sealant between the toughened glass and the aluminium frame. This information comes from one of the top panel manufacturers, and is likely to apply to all makes of panel. It is, however, something that could be examined during an annual check. In general PV panels are very long-lived and the oldest have been working in space for 55years! For other panel details, refer to the appropriate energy generation section above; or contact TigerGreen directly.
  • Solar Panels in the Summer 2011 issue of Practical Boat Owner magazine. This is a brilliant article on the use of solar panels teamed up with battery storage for use on boats. It is written by a do it yourself enthusiast who lives on his own sailing cruiser and uses the power to provide for the niceties of daily life. Apart from the excellent graphs and his case study, he mentions two other very useful bits of information. For all off grid systems, whether they be on a boat or a house, it is essential to have a Regulator. The regulator's job is to avoid the power output from the solar panels from damaging the batteries by over charging them, when the panels are charging under peak conditions.
  • The following website provides essential initial weather data for those thinking of going into either solar or wind energy. The author, of the above article, explains how to use the NASA website, but the url given does not seem to work reliably. After playing around with Google I have found that the following Link does work. It opens on a page with a link to the SSE Website at the beginning of the second paragraph. This takes you to a page with "Meterology and Solar Energy" at the top of the left hand side. Click on this to take you to a page offering a search on either your desired map location or your Latitude & Longitude e.g. 50.6 and 3.3. You will be asked to register with your name and telephone number, but this is free. This takes you to a page with all the data you could possibly want with monthly averages over 22 years and the data for SSE HRZ is probably the best place to start. For those of you without a Practical Boat Owner, I will summarize some of the most relevant information in the Solar PV section.
  • When putting up Photo-voltaic panels (PV) on your roof or shed, it is worth bearing in mind that: although the sales persons are not lying when they say that PV panels can generate electricity in any light, even moonlight; what they are not saying is that your expensive new PV system cannot generate at its peak rating if it is not sited to get maximum light. Throughout northern Europe manufacturers advise siting their panels only on roofs or stands facing from south-east (135°) through south to south-west (225°). For maximum production most manufacturers prefer their panels to be placed between SSE and SSW.
  • Another way in which electrical generation can be maximized is by ensuring that PV panels are installed at an angle of 45 - 60° from the horizontal, depending on how far south or north you are in the UK. Basically the angle needs to be the same as the latitude of your home. Some installers have now accepted that the elevation of the sun changes between summer and winter; therefore, it is beneficial to have a system that allows you to increase the angle in winter. If feeding directly to the grid this last suggestion is not critical; but, it is if your house is in an isolated spot and you need to maximize your winter production to keep your batteries/accumulators topped up. For other tips, see the Energy Generation section.

Please mention TigerGreen whenever you contact any of our linked Top Providers, Suppliers and Contractors.