Model limitations
What are the model limits?
The Solar Wizard tool helps individuals, local authorities and community groups assess if their building(s) are right for solar photovoltaic (PV) panels. Unfortunately, the tool isn't perfect - the real world is just too complicated. But that doesn't mean it's useless! It just means there are a few points you should be aware of to make sure the results are useful to you.
Economics
The tool offers several options to fine-tune its economic calculations to your needs. It's important that you set these to represent your own circumstances. The better you represent your life, the more useful the results will be. However, there are a few costs that the tool does not cover:
Limits in the model
- Scaffolding costs - this is a major additional cost. Installers must access your roof to install panels. To do so, they will have to build scaffolding. It's hard to estimate the cost of scaffolding because it depends on a lot of things, including the height of your building. As an indicator, prices are often around £500-1000.
- Perimeter access costs - in many ways, this is related to scaffolding costs. Your building may be next to a road, in which case it is easy to access, which is great! But maybe you live at the end of an unpaved farm track. Installers may have to spend more to access your property, and they may pass those costs along to you!
- Roof state - there is no way to assess the condition of your roof without visiting. If your roof is weak or damaged, then there will be additional costs to repair this damage before PV can be mounted.
- Wiring costs - installers must wire your panels into the electricity grid and your home electrics. The length of the wire for this can vary considerably, affecting costs. The net result of this, is that while the tool will give you a reasonable estimate of the costs, it does not give the complete picture. The best way to think of this, is that the tool gives you an initial estimate, which can then be made more accurate through on-site assessments by an installer.
The horizon shading profiles are derived from the elevation map. Because of this, the shading profiles only include things larger than the resolution of the elevation map. This resolution is 1m. The horizon profiles are also split into 36 slices (each slice covers 10 degrees). This means that, the profile can account for things like buildings and hills but are unlikely to account for smaller objects like trees and lampposts. The tool should be compared to what you can see in the real world to understand if there may be additional shading.
The tool estimates the roof “planes” - the flat surfaces that make up the roof. For example, a gable roof would be made up of two planes - one each side of the roof peak. Planes below a minimum size are excluded, so very small or complex roofs may not be considered. Then, for each plane, the tool tries to fit as many panels in as it can - in either a landscape or portrait orientation. Because the orientation is not mixed, an installer may manage to fit more panels than suggested by the model by mixing portrait and landscape-oriented panels.
Panels mounted on a flat roof will need to be mounted on a frame. The model considers them as being mounted on frames at 10 degrees. The best angle would be between 30-40 degrees, which increases the electricity generation for each panel, but costs more - because they need more ballast. More steeply angled panels also require more spacing between them, which can decrease the overall electricity generation.
