First, a brief history

Photovoltaic power is a French discovery: it was found in 1839 by the Frenchman Antoine Becquerel

Some physics

Solar energy hitting the earth has an average power of 1000W/m² but solar energy is distributed very unevenly over the surface of the globe, tropical areas and deserts receive overall more energy than the poles.

The main reason for this inequality is that the earth’s surface is round. In the areas near the equator, the sun shines almost perpendicularly down onto the surface of the earth, meaning that each square metre of ground surface captures practically the totality of the sun’s rays.


 ( Light loses almost 75% of its intensity as it goes through the atmosphere!)


In Polar Regions, the lighting conditions are a lot less favourable, the sun’s light reaches the earth at an oblique angle compared to the horizontal, the same quantity of energy is thus spread over a greater surface lessening the intensity per square metre. What’s more, the light has to travel a greater distance through the atmosphere, so its intensity is proportionally weaker.


The second reason why the sun’s rays are less powerful is the presence of clouds that in some areas reduce even further the intensity and the duration of the annual amount of sunshine. All these facts mean that exploitable solar energy can vary by a factor of ten, depending on the location under consideration.

How do we measure solar energy?

The unit of measurement of solar energy is expressed in kilowatts per hour per square meter per day (Kwh/m²/day). In France, a square meter that receives as much sun as is possible will receive the maximum power of 1000W.

In one hour this square meter will be able to accumulate 1Kwh of energy.

However an optimal amount of sun won’t be received consistently throughout the day: morning and evening the sun’s angle will mean the rays are weaker, not to mention the possibility of clouds! Over the whole of the day we will only be able to obtain 5kWh/m²/day in Nice in the heart of the summer, and a mere 700Wh/m²/day in Strasbourg in winter.



It is thus important to differentiate between the maximum possible solar energy in the middle of the day in summer, and that of an ordinary day.

A solar panel produces electricity intermittently, according to the amount of sun it receives. The electrical power it supplies expressed in kilowatts (KW), varies therefore depending on where you are, the time of day and the season. In order to be able to compare the power of photovoltaic installations, the unit of measure is the KWp this being the maximum power an installation can provide in optimal lighting conditions (1000W of light /m², which in France corresponds to the sunlight at midday and a south-facing solar panel).


Now let’s talk about ecology


An installation of one photovoltaic kilowatt avoids a yearly emission of 600Kg of CO² compared to an equivalent production of electricity from fossil energy sources, which is a great deal more that the quantity of carbon mono-dioxide liberated during its manufacture. Even in France, where 80% of the energy is nuclear, each photovoltaic kWp installed is considered to save, over 20 years, 3.5 tonnes of CO² and 500 grams of high activity radioactive waste.