How to choose an efficient solar panel

Efficiency rating calculations

The efficiency of a solar panel is calculated by dividing the maximum power output (measured in peak watts or Wp) by the amount of sunlight absorbed by the panel.

Indicated as a percentage (%), the efficiency rating of a solar panel represents the amount of sunlight it can turn into electrical energy. The higher the percentage, the more efficient the solar panel.

Efficiency and panel size

When comparing two different panels of the same size, it goes without saying that the panel with the higher efficiency rating will produce more electricity. The panel with the lower efficiency rating will need to be larger than the more efficient panel to produce the same amount of energy. If you have limited space – for example, if you have a small roof or you are looking for very powerful panels to power a camper van – you will need more powerful solar panels. But bear in mind that these panels will also be more expensive. If you don't require a huge amount of power and you don't have any space constraints, you can purchase less powerful solar panels which will also cost you less.

The photovoltaic cells that make up solar panels work using different technology. Some PV cells are over 60% efficient. Unfortunately, most of this technology has been developed for very specific purposes, such as for spacecraft, and is not available to the general public. The types of solar cells available to the public are made using silicon. The efficiency of the cell depends on the method used to manufacture the crystalline silicon and can be as efficient as 25%. There are three different types of silicon cells.

Amorphous silicon

These cells can produce up to 60 Wp/m² meaning an efficiency rating in ideal sunlight conditions of around 6%. Inexpensive to produce, these types of cells are used for small electrical devices like watches or calculators. The flexibility of these cells means they can lend themselves to a range of applications.

Monocrystalline silicon

This type of cell can produce up to 150 Wp/m² for an efficiency rating of 16 to 24%. These PV cells are formed of a single silicon crystal that is cut into thin sections and can be recognised by their uniform blue surface. This manufacturing process makes these cells expensive. What's more, the efficiency of these solar panels will drop if sun exposure is not optimum or if they are exposed to diffuse lighting.

Polycrystalline silicon

These PV cells provide an efficiency rating of around 15% and can produce up to 100 Wp/m². The silicon used to form these cells goes through a melting process. As it cools down, a number of crystals form which gives these cells a range of blue tones. The efficiency of these cells is also affected by low light conditions and diffuse lighting.

The installation site of your solar panels is of the utmost importance. Efficiency losses of up to 30% can occur if the orientation and tilt angle of the panels is not optimal. You can buy solar panels that are very efficient on paper but they won't produce as much energy as expected if they are installed incorrectly. The ideal orientation for solar panels is full south-facing for maximum production.

A south-east or south-west facing solar panel will experience losses of about 4 to 5%. A panel facing east or west can have losses of up to 20%. Ideally, solar panels should be tilted to 30 to 35° from the horizontal. A solar panel installed flat will see efficiency reduced by 10% while vertical installation will incur losses of up to 30%. In short, choose your installation site very carefully to ensure your solar panels work to their full potential. And, of course, don't forget to clean the panels from time to time!

• Solar panel buying guide

• How to calculate solar panel size and output

• How does a solar panel work?

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