Rātā Chapman Olsen, Geospatial Data Scientist
Over the course of this year we have been modelling roof top solar power generation across our towns and cities from a Harrisons Energy 3 KW solar panel system. To calculate this, we use Lidar data to model solar potential city-wide considering the size, angle and orientation of roof tops as well as shadows from nearby trees, buildings and hills.
We then intersect the solar surface with building outlines to determine a unique solar receipt heatmap for every rooftop. From this we can predict the likely placement of a solar array and estimate the power generation potential in kilowatt hours and dollar terms. These are visualised and available free on homes.co.nz.
We have modelled over 1 million residential and commercial roofs across New Zealand including those in our three largest cities – Auckland, Wellington, and Christchurch.
To examine the solar potential across the housing stock of each city, we plot the estimated power generation per house over percentile, so that the houses with the least potential per city are on the left of the graph below and those with the highest solar potential are on the right.
Looking at the graph we note the strong influence of latitude which determines the position of the sun with respect to the land surface. Auckland (blue) has a latitude of approximately 37ºS, Wellington (yellow) 41ºS while Christchurch (red) has a latitude of 43ºS. As a result of the solar declination, Auckland roofs generally speaking receive a higher solar input over the course of the year.
This variance is more pronounced in the first tenth percentile or so for each city. A low yielding property in Auckland and Christchurch returns approximately $800 of power while in Wellington the figure is only $600.
Overall, the curves are reasonably flat over the remaining properties in Christchurch and Auckland. This suggests topography or local shadow impacts in general are not substantial in these cities for all but 10 -15% of properties. We do note also an interesting upward inflection for the upper most yielding properties in all cities.
Wellington City however shows a different trend line. Nearly 40% of all roof tops here are impacted by shadows resulting in solar receipt loss. In these properties the annual power generation falls below the equivalent percentile properties in Christchurch – a city which lies some 300km to the south. We assume roof pitches, materials and construction methods are similar across all three cities.
We have visualised this further using a hillshade model from all three locations (below).
Auckland hillshade
Wellington hillshade
Christchurch hillshade
The images reveal that Auckland and Christchurch are relatively flat for the most part while Wellington has a much larger level of physical relief across the city area. The table below further shows the elevation range, mean and a single standard deviation in the urbanised city areas. We again find that Wellington has a far greater mean elevation and variation across its housing stock.
Elevation range, mean and standard deviation across the cities
We would expect there to be some secondary effects from vegetation or building density but in broad terms, these impacts are dwarfed by the physical relief profile.
Overall, the data reveals that while latitude is important it is not always the sole determinant of solar investment return. Understanding all local shadow impacts including topography, tree canopy and buildings is important in determining the returns from a solar investment.
Talk to your installer also as panels may be able to be positioned on a roof with an adjusted tilt angle to counter local shadow impacts. To find the individual solar generation potential of your home go to www.homes.co.nz.