The solar installers inherited a rule of thumb from the satellite-dish engineers – tilt up your panel to the same angle as your latitude. For Bristol at 51.5° North that would be around 51° to 52°. That seems to be a very simple rule and looks right for Bristol but is slightly wrong.
The sun is not at the same angle in the sky throughout the year, it’s at its highest in midsummer and lowest in December, which is why a solar panel, optimised for collecting the sun’s energy in midsummer, will lose most of its efficiency in the winter months. A year round average angle of tilt for a solar panel for Bristol would be a degree or so less than the latitude of the site.
But it is overcast days, where much of the UK’s solar energy is gathered, as scattered light or diffuse radiation, that this matters more. The Met Office describes how diffuse and direct solar radiation reach the ground and whilst a large amount of this radiation is scattered in all directions by the clouds, a significant amount of it makes it down to the ground. Diffuse sky radiation is an important component of the total solar irradiance reaching the Earth’s surface, and in a country as overcast as the UK, it makes up a large proportion of the total irradiance throughout the year. Thus, a lower pitched roof, collecting radiation scattered in all directions from the whole of the sky, rather than from a small patch of directly shining sky, is more efficient. There is more on Solar Panel Installation Bristol at //redbridgeandsons.co.uk/solar-pv-panels/solar-panel-installation-bristol/.
Of course, there is also the matter of the Victorian rooftops in Bristol’s older terraced streets, such as those in the Clifton and Harbord areas. These rooftops generally have a quite steep pitch, but not so steep as to be in the higher end of the possible, and that typical pitch of 35° to 40° is within the annual optimum for solar collection for our latitude.
While solar panels in Bristol rely on the grey skies for most of their energy, they are actually collecting energy from the whole sky, not just from the sun.
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