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Solar (electric) Power


Solar Electric
Image: Corny

Solar power is produced by conversion of energy from sunlight into electricity, most commonly using photovoltaics (PV) panels.

Photovoltaic panels use silicon to covert solar energy into electric. The PV solar cells act as a semiconductor which absorb sunlight and transfer the energy to negatively charged particles (electrons) that flow through the semiconductor as electrical current.


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Introduction

Photovoltaics (PV), or Solar Electric, work by converting the sun's energy into electricity using roof-mounted panels. Cheaper units convert some 5 per cent of solar energy into electricity and more efficient, and more expensive units, convert up to 18 per cent of energy received into electricity.

Power output depends on the type of materials used in construction and the amount of sunlight received. The maximum output from PV systems is in the summer, but the maximum power usage in a property is in mid-winter. Energy from these systems can be sold back to the National Grid.

A maximum output energy rating for panels can be misleading and does not relate to the amount of resource (sunlight) that is actually available at any one location. A better way of comparing panels is their % efficiency; this tells you what % of the light landing on them they will turn into electricity. This combined with a figure for the average solar radiation at your site and the size of the panels will give you a far better idea of how much electricity you will generate.

See Homemicro's information on photovoltaic elecricity generation here

Rule of Thumb

The following are approximations which can be used for preliminary scheme selection.

  • • 1kWp photovoltaic array, measuring at least 7.5m2, will generate ≈750kWh per year in the UK climate.
  • • Carbon dioxide savings for 1kWp will be 480kgCO2 per year.
  • • Average power is 20% of peak power, so peak kilowatt of solar array output power corresponds to energy production of 4.8 kWh per day (1kWp x 24hours x 20%).
  • • Expected annual energy yield (kWh) = Panel Efficiency (%) X Average annual solar radiation (kWh per m2) X Area of panels (m2).
  • • 1kW Mono-crystalline PV solar panels fully installed starts from £7,979.
  • • A typical low-energy house will need 3kWp to 4kWp capacity.

Further Information

Useful Reading

CIBSE: KS15 Capturing Solar Energy

PV Schematic

Click on the image below for a schematic diagram of a solar electric installation

solar electric schematic diagram

Factoid

PV panels output is quoted at 25°C, for every 1°C hotter the operating voltage and power will reduce by about 0.5% & also overall efficiency decreases by up to 1% per year.


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