How to Estimate Solar Efficiency

How to Estimate Solar Efficiency

To predict solar efficiencies for homes, the Government refers to a Standard Assessment Procedure (SAP) calculation. Regional variations, nationally and internationally, have been calculated with information obtained from Photovoltaic Geographical Information Systems (PVGIS) from around the world.

The degree to which the atmosphere affects efficiency is related to the turbidity (or transparency) of the sky, and for that particular region. This is the Linke Turbidity rating or TL value.

Interactive information generated from knowledge of the sun’s energy, the geographic distribution, and the limiting factors, are collated into databases. Using that information, very specific calculations can be made for the amount of energy potentially generated at any given location.


SAP is the Government’s Standard Assessment Procedure for calculating the energy rating of UK houses and is a requirement for all new-build homes.

The potential efficiency of a solar installation on a property, anywhere in the UK, comes, in part, from an assessment of roof orientation, slope and shade. This is measured using a SAP calculator, but that doesn’t fully account for regional (or latitude) variations.

Photovoltaic Geographical Information Systems (PVGIS)

PVGIS compares latitude variations and compiles information from databases around the world. These values represent the monthly/yearly average amount of solar radiation energy hitting one square meter, in a horizontal plane, in one day (Wh/m2/day). These factors help calculate optimum inclinations when designing domestic solar systems.

Linke Turbidity Factor (TL)

Atmospheric turbidity is simply a measure of how clear and dry, or transparent, the sky is. Solar radiation reaching the earth is affected by factors such as water vapour and aerosol particles. This ‘turbidity’ of the atmosphere is referred to as the Linke Turbidity (TL) value, and is specific to regions

A clean, dry, blue sky would have a TL factor of 1. This would be nearer 3 with increased water vapour, and whiter skies. Over a busy city, where there is much greater turbidity from pollution, the value can rise to 6 or 7

Europe’s TL average is 3

Solar power is a clean efficient means for houses to generate up to a third of the home’s electricity using renewable energy, whilst also benefiting from a generous annual Government incentive, interest linked and paid to you for 25yrs

Leave a Reply

Your email address will not be published. Required fields are marked *