Earlier this year, Pager Power began offering a new service to developers in the form of Daylight Sunlight Assessments. This included the development of our own bespoke model that complied with BRE and British Standards guidance to ensure accuracy and compliance with best practices. More information on the guidance can be found here.
This series of articles looks at the technical terminology used within these assessments, with this specific article exploring the term ‘Annual Probable Sunlight Hours’ or APSH. Other articles in this series look at other terms, including ‘Vertical Sky Component’ and ‘No Sky Line’.
Definition of Annual Probable Sunlight Hours
Simplistically, Annual Probable Sunlight Hours correspond to the expected number of hours of sunlight at a location based on latitude and climate. APSH is an important reference when assessing the impact upon sunlight in a Daylight and Sunlight Assessment.[1]
The way that APSH is typically calculated in a Daylight and Sunlight Assessment is through the use of a Sunlight Availability Indicator, as shown in Figure 1 below. This indicator is for London, and each of the blue dots approximates 0.5% of APSH (from a total 1486 hours).
Figure 1: Sunlight availability indicator for London (51.5°N) [2]
What Does the Guidance Say?
The BRE 209 guidance states that APSH should be used to quantify the effect of overshadowing on existing dwellings and “non-domestic buildings where there is a particular requirement for sunlight” [3].
The guidance applies thresholds where rooms should receive at least 25% of APSH and at least 5% of APSH between the equinoxes. It further states that ‘any reduction in sunlight below these levels should be kept to a minimum’ [4]. Finally, as with VSC, efforts should be made to ensure that the value of APSH is at least 80% of the baseline.
It is important to note that APSH calculations are only required to be completed for windows which face within 90° of south and where an obstruction from the new development is located south of the window, within an assessment area, which measures to three times the height of an assessed proposed development. [5]
How is APSH Modelled?
Pager Power uses the results of the Vertical Sky Component assessment (see here for modelling details) and overlays the results from the skylight indicator upon one of the sunlight indicators (dependant on the latitude of the development).
More information on Pager Power’s Daylight and Sunlight Assessment modelling can be found here.
Speak to an Expert
If you are interested in finding out more about our Daylight and Sunlight assessments, follow the link here to make an enquiry, or give us a call on +44 (0)1787 319001.
About Pager Power
Pager Power is a dedicated technical consultancy that has been providing independent guidance and advice regarding solar developments, wind farms, and building developments internationally since 2002. Further details about the services we can provide can be found here. Pager Power has completed over 1,400 glint and glare assessments, over 1,000 aviation/radar impact assessments, over 500 television and radio reception surveys, and over 500 telecommunications impact assessments.
References
[1] Paul J Littlefair et al, ‘Site layout planning for daylight and sunlight’ (BRE 209, 3rd Edition 2022), Building Research Establishment
[2] Ibid, Figure A2
[3] Ibid, 3.2.1
[4] Ibid, 3.2.7
[5] Ibid, 3.2.9
Thumbnail image accreditation: Andrej Lišakov (Sept 2023) from Unsplash.com. Last accessed on 29th September 2024. Available at: https://unsplash.com/photos/a-plane-flying-over-a-city-with-tall-buildings-uAUE8ic3O7w
