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How Do Solar PV Panels Work

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How it works, in brief:

Solar Panel Efficiency

Photovoltaic systems use cells to convert solar radiation into electricity. The PV cell consists of one or two layers of a semi conducting material, usually silicon. When light shines on the cell it creates an electric field across the layers, causing electricity to flow. The greater the intensity of the light, the greater the flow of electricity.

Photovoltaic (PV) panels generate electricity, which can be used in your home. During installation of a PV system, there is little disruption to your home - all the components are on your roof or in your loft, apart from a meter and switch which can be installed by your main fuse box.

Diagram showing the total average solar radiation falling on one square metre surface inclined at 30 degrees to the horizontal, measured in kilowatt hours.

UK Radiation Map

Lower Electricity Bills

Your electricity bills will be lower because often you will be using electricity from the PV system instead of taking electricity from the mains. A typical household consumes 3000-4000 kWh on average per annum. A 2 kWp system would provide almost 50% of your electricity needs i.e. 1500 kWh. This can vary considerable dependent on your electricity consumption and how energy efficient your property and electrical appliances are. It is possible for a 2 kWp PV system to supply all your electricity needs if you implement all possible energy saving measures and of course with a larger system 3-4kWp you could easily meet your needs, but you would obviously need more unshaded area and a larger budget.

How much will a grid connected system cost?

Prices for Solar PV systems vary, depending on the size of the system and the type of cell used. Systems installed on top of your existing roof are less expensive than systems integrated into new build. Solar tiles cost more than conventional panels, but the cost of tiles that would be displaced can be offset against the Solar tiles. The size and price of your system will depend on the suitable available roof area and your budget.

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The components typically required in a grid-connected PV system are illustrated below.
Typical Components used in a Solar Panel System

The PV array consists of a number of individual photovoltaic modules connected together to give the required power with a suitable current and voltage output. Typical modules have a rated power output of around 75 - 120 Watts peak (Wp) each. A typical domestic system of 1.5 - 2 kWp may therefore comprise some 12 - 24 modules covering an area of between 12 - 40 m2, depending on the technology used and the orientation of the array with respect to the sun.

Most PV modules deliver direct current (DC) electricity at 12 volts (V), whereas most common household appliances in the UK run off alternating current (AC) at 230 V. An inverter is used to convert the low voltage DC to higher voltage AC. Numerous types of inverter are available, but not all are suitable for use when feeding power back into the UK mains supply.

Size of System - kW
Approximate Yearly output - kWh*
*Output will depend on the weather and type solar panel - 1kWh = 1 unit on your meter


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We also offer a range of DIY kits for remote buildings such as stables barns and out-houses which can be installed easily. These systems are generally mounted on an existing roof or placed in an un-shaded area in an array of solar modules and come with full installation details. Solar Power Station


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More About Solar Panels

Solar Panel Efficiency

First let's consider what we mean by efficiency. Efficiency is a measurement of the output divided by a certain factor. When talking about solar cell or panel efficiency, we are gerally looking at output per given area.
Therefore a more efficient panel will either give more power or be smaller than a less efficient one. This does not mean that a more efficient panel is more economic, in-fact the reverse may be the case.
More efficient panels may be easier to mount, particularly if space is limited, or a tracking system is being used.

There are several separate areas to consider here concerning the efficiency of pv solar panels, concerning the inherent efficiency of the cells in the panel you buy, the panel construction, and how your installation may affect panel efficiency.

For home owners setting up there own system, efficiency is likely to be less important than the cost per nominal watt output.

Solar Cell Efficiency
Researchers are continually striving to improve the efficiency with which solar cells convert light energy to electrical energy. The record at the moment stands at an efficiency of around 40%, using multi junction cells (multiple layers of silicon), each layer tuned to trap different frequencies (colours) of light. This type of cell will however be expensive to produce and in the past has been mainly used in space where efficiency may be more important.
Researchers also strive to increase the amount of light entering the cell. Silicon is a naturally shiney substance and cells are coated with non-reflective layers to ensure that as much light as possible enters the cell.
Cells used in photovoltaic panels for electricity production are usually single junction type with an efficiency of somewhere around 15%.

Solar Panel Construction
Solar panels will normally have a layer of glass (protecting the cells) through which the light must pass before entering cells. There is scope for reducing light reflected from the glass and also to trap light that is not striking the panel at 90o.
Some research centres on trapping light from a large area from various angles and directing it onto a relatively small cell.

Panel Mounting and Positioning
The two important factors here are the angle at which the panels are mounted and the amount of sunsine falling on the particular location/avoidance of shade. Small areas of shade on a panel can have a significant effect on the power produced.

Effects of Temperature
The output of a solar cell, and therefore a solar panel, is affected by its temperature. As a result the power output will be reduced by between 0.25%(amorphous cells) and 0.5%(most crystalline cells) for each degree C of temperature rise.
Panel temperatures in the summer in warm climates can easily reach 50oC resulting in a 12% reduction in output compared to the rated output at 25oC.
This reduction in efficiency may be important to you if you have a high electricity demand in the summer.

Partial Shading
Solar panels obviously produce less power when they are shaded and should idealy be situated where there wil never be any shadows on them.
There may be situations where this cannot be avoided, and the effects of partial shading should be considered.
A shadow falling on a small part of a panel can have a surprisingly large effect on output. Not only will the cells that are shaded be producing less power, but as the cells within a panel are normally all wired in series, the shaded cells afffect the current flow of the whole panel.
If the affected panel is wired in series (in a string) with other panels, then the output of all those panels will be affected by the partial shading of one panel. Therefore in a situation where partial shading cannot be avoided, there may be a case for not having the panels wired in series to produce the higher voltages that can be used with some inverters.

How Your System Affects Panel Efficiency

Apart from positioning and angling your panels in order to capture as much light energy as possible, there is more.
The inherent characteristics of solar cells results in current produced by a particular light level being virtually constant upto a voltage of 0.4 volts. A solar panel with a nominal voltage of 24 volts would normally have 72 cells, resulting a constant current upto 28.8 volts. Above this voltage, current drops off rapidly, resulting in maximum power output being produced at around 28.8 volts.
When the panel is connected to the battery via a simple charge regulator, the voltage will be pulled down to near that of the battery. The result is that the panel will only be able to produce it's maximum power when the battery is near to being fully charged.

This is where an MPPT (Maximum Power Point Tracking) Charge Controller can play a part by maintaining the panels at their optimum voltage while producing the voltage required by the battery.

What about if you are not using an MPPT controller? Well if you are in a situation (possibly in winter) where your batteries rarely become fully charged, adding an extra panel will not only add the extra rated power potential, but the rest of your panels will be more efficient asthey will be running at a higher voltage.
This effect will to some degree be negated by the battery being less efficient at higher levels of charge but remember that it is beneficial to battery performance and life to be fully charged on occasions.

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Solar panels are large flat objects that may catch the wind and they are expensive - so they need to be securely fixed. Also, considering their value, you may want top make them difficult to remove!

Which Direction Should They Face
In most cases, if you are in the northern hemisphere they should face due south and if in the southern hemisphere, they should point due north.
You may need to vary this slightly if you cannot avoid a hill or high trees etc blocking the sun at one end of the day. If hill to the west results in you losing the evening sun early, then use your judgment to point your panels SLIGHTLY towards the east.
I emphasise slightly as it must be remembered that the early morning sun will be of little value as it is so low in the sky.

The Angle to the Horizontal
The general advice here is that the angle of your panels relative to horizontal should be the same as you latitude in spring and autumn, 15 degrees less in summer, and 15 degrees more in winter.
If at all possible, it will be worth mounting your panels so that you can change their angle at least 4 times per year.
If you have to mount them so that they are fixed then go for the same angle as your latitude. You may however wish to vary this according to which part of the year is your need the greatest. For many people this will be winter time though if you have swimming pool and cooling fans for the summer,this may not be the case.

Materials Used
Galvanised angle steel is usually appropriate. There is a slight possibility of corrosion occurring between aluminium (the normal material used for the frame of the panel itself) and the zinc used in the galvanising. However in most outdoor situations that are reasonably dry, this shouldn't be a problem.

Panels will be much more efficient at producing electricity if they are always facing the sun. Trackers are available that serve as the panel mounting and will point them towards the sun.
There are several types to choose from, Passive trackers use a system whereby a liquid moves as it is heated by the sun and is used to move the panel, automatically returning to the correct position for the morning. This type is likely to be the cheapest, doesn't have much to go wrong, and is reported to give satisfactory results.
Active trackers will usually use electric servo motors to angle the panels and these may be controlled by a light sensor sensing the position of the sun or may be controlled by a time moving the panels to predetermined positions.

The increased production of electricity that you will get as the result of using a tracker will be greater during the summer than the winter due larger arc taken by the sun across the sky in summer.
This benefit may be mostly when you least need it and you may feel that the money would be better spent on a couple of extra panels.

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