Solar panel buying guide

3 factors for choosing a solar panel

When it comes to choosing a solar panel, you will have to evaluate your electricity needs and consider the natural and physical limitations of your location. These factors can be summed up as follows:

1. The kWh required throughout the year (depending on your electrical appliances and usage).

2. Sun exposure throughout the seasons where you live.

3. The size in m² of the space you have for installation which affects the type of solar panel technology you can use.

3 types of solar panel technology

There are three types of solar panel technology, each of which offers different performance and features:

1. Amorphous silicon: 1 m² = approx. 60 Wp; works in diffuse light. Efficiency = around 6%.

2. Polycrystalline silicon: 1 m² = approx. 100 Wp; performance is lowered in diffuse light. Efficiency = around 15%.

3. Monocrystalline silicon: 1 m² = approx. 200 Wp; performance is lowered in diffuse light. Efficiency = around 16 to 24%.

The size of your overall installation should be worked out according to the size of the panels, which can vary depending on technology (regardless of peak power). To give you an idea, one kWp (i.e. 1000 watt peak) can produce 850 kWh in a region with an average number of sun hours and 1250 kWh in a region with a high number of sun hours.

Components of a solar panel

Solar panels are made up of photovoltaic (PV) cells a single one of which can produce about 0.5 V. These cells are set up in series or parallels to form a panel with an output voltage of about 12V and an average of about 6 amps per sun hour.

The PV cells are surrounded by an encapsulant and connected to conductors, making it possible to gather the electric current created by the cells when they are exposed to sunlight. The whole assembly is covered with a transparent plate which is usually made of tempered glass so as not to inhibit sun exposure.

These layers are protected from impacts by a frame which is used to securely fasten the panel to a roof or on the ground using a specially designed frame. The panels must not move, even in high winds.

The cells that make up solar panels are made of semi-conductive materials like those you might find inside a computer. At the heart of the cells is what is known as a p-n junction diode. On one side, you have a P-type material with a deficit of electrons (meaning it is positively charged); on the other is an N-type material with an excess of electrons (meaning it is negatively charged).

The combination of the two creates a permanent electric field. The energy contained in particles of solar energy, known as photons, energises the electrons of the semi-conducting materials that make up the cells. Once the electrons are sufficiently energised, they come loose from the atoms and are ejected by the electric field created by the p-n junction.

The power of a solar panel is indicated in watts peak (Wp). This value refers to the maximum output power provided by a solar panel at 25°C under standard sunlight conditions using a solar radiation standard of 1,000 watts for every square metre of panel.

Peak power is used to compare different solar panels. The higher the Wp, the more electricity is generated. Finally, this value can be used evaluate the annual performance of a solar panel installation depending on its installation site.

To give you an idea, one kWp (i.e. 1000 watt peak) can produce 850 kWh in a region with an average number of sun hours and 1250 kWh in a region with a high number of sun hours.

Photovoltaic cells can function using different technologies. This technology impacts the price and performance of a solar panel. The better the performance, the higher the peak power and the more electricity can be produced annually. Bear in mind that the most efficient solar panel technology is generally reserved for advanced scientific applications.

Three types of solar panels

Amorphous silicon

One metre squared (1 m²) of these panels produces 60 Wp for an efficiency of around 6%. In addition to being affordable, these panels can even function using diffused light and can be installed on flexible stands which is handy if you plan to move the panels around.

Polycrystalline silicon

The peak power of these panels is around 100 Wp per square metre (m²) for an efficiency of around 15%. Looking at the cells, you'll notice that they feature different tones of blue. These panels are less efficient in cloudy weather.

Monocrystalline silicon

More complex to install than the panels mentioned above, these panels are also 50% more powerful with an efficiency of 16 to 24% and peak power of approximately 165 Wp / m². These panels are sensitive to diffuse lighting which will lower performance.

Solar panels vary in size depending on several parameters each of which must be considered before purchase. However, you may be able to play around with certain factors depending on your aims:

• peak wattage in Wp per m² of the solar panel: this depends on the technology you choose and the specific performance specifications of the model;

• the amount of electricity you hope to produce over the course of a year in kWh;

• the level of sun exposure available, and your geographical position. A m² of photovoltaic cells won't receive the same amount of sun in the north of the country as they would in the south. -

• the size of the area you have available to install your PV panels;

• the orientation of the panels: ideally they should be south facing and set at a 35° angle from the horizontal. The further your panels get from this optimised set-up, the less sun exposure they will receive and the less electricity they will produce. A panel installed perfectly vertically or horizontally will never produce as much as it could if installed at the correct tilt angle.

Follow our guides for more advice on how to size a solar panel.

1. For a stationary installation, the most important factor to think about is how much power you can get out of the space you have available.

2. Highly efficient solar panels will produce more per m² and are therefore more expensive to purchase. These panels feature more advanced technology and special care has been taken to manufacture them.

3. To compare two types of solar panels objectively, the best thing to do is to look at the Watt peak per square metre (Wp/m²).

4. Work out your calculations with a precise idea of electricity production in mind taking into account all the parameters of your installation. The formula we have provided allows you to evaluate the panels in terms of annual production with peaks of production in summer making up for lower production in winter.

5. Do not hesitate to factor in a large margin if you want your panels to provide power all year long. This will help to ensure that you are never short of electricity from summer through to winter.

6. Don't forget that your installation site is of the utmost importance. Be sure to check that no shadows will be cast on your panels. Remember that any surrounding trees will grow and take care to clean the surface of your solar panels after installation to guarantee good performance.

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• How to calculate solar panel size and output

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