Solar Photovoltaic Energy is the transformation of sun light in electric energy, that is done using solar photovoltaic cells, assembled in solar modules/panels. Photovoltaic systems can be classified as autonomous or grid-connected. In the former, all energy produced is consumed by the producer.

Grid-Connected Photovoltaic Systems

In Grid-Connected Photovoltaic Systems part, or none, of the energy produced is consumed by the producer and the rest is delivered to the electric grid, generating yield for the producer.

A Grid-Connected Photovoltaic System is composed of a set of solar modules/panels, one (or more) inverters and a energy meter.

In Portugal, there is an interesting feed-in tariff for selling renewable energy to the electric grid, known as "Microgeração" (microgeneration). 
The revised law (DL nº363/2007) defines a feed-in tariff of 0,40€ per kWh during the first 8 years and 0,24€ per kWh for the next 7 years. These tariffs will decrease with time. Be the first to move and get the best tariffs.

Connecting a solar photovoltaic system to the grid is currently a very interesting investment, with interest rates of 9%-10% (25 years) and payback in 7-9 years, at a low risk (mature and reliable technology).

Autonomous Photovoltaic Systems

In Autonomous Photovoltaic systems all energy produced is consumed by the producer.
Autonomous Photovoltaic systems differ from Grid-Connected systems in the following:

1. They need to be sized regarding energy consumption
2. They need bateries to store energy. Energy is produced in excess during the day in order to be available during night and cloudy times. System autonomy and available power will depend of the overall system sizing.
3. It may be necessary (non-renewable) support systems (ex: diesel generator)

From the environmental viewpoint these systems are very interesting since most part (or all) electric energy is produced from renewable sources.

For the same power, Autonomous systems are far more expensive than Grid-Connected systems, and they can be financially interesting for remote houses that are far away from electric grid.

Key Factors to take into Account

The main factors to take into account when buying solar photovoltaic systems are:

• Always choose certified solar modules (certifications: IEC 61730 and IEC 61215 for crystalline modules and IEC 61730 and IEC 61646 for thin-film modules).
• Choose modules with minimum peak power warranty not below 90% of module peak-power during the first 10 years and 80% of module peak-power for the 11th year until the end of the 25th year.
• Solar modules produce less with temperature rise. Choose solar modules that deals best with temperature.
• In order to have your system fully optimized, it's important to "fit" solar modules with inverters. Don't go for the cheapest. Choose experienced installer companies.

Learn more about:

• Photovoltaic Technology