Most solar estimates assume an open sky. Real balconies are not open sky, they sit behind other buildings, under trees, facing the wrong way. The shadow simulator exists to put your actual surroundings into the estimate, so the number you see is closer to what you will really get. It combines four inputs, each chosen because it is either exact or independently sourced.
1. Where the sun is
For any date, time and location, the position of the sun is a solved astronomical problem, its azimuth and elevation can be computed exactly. This part is not modelled or estimated, it is calculated, and it is the backbone the rest of the tool hangs on. It is also why the tool can play a whole day and draw a year-round sun-and-shade pattern for your spot.
2. What is around you
The buildings near your balcony come from OpenStreetMap, the open global map database, including their footprints and, where recorded, their heights. The tool builds these into 3D and works out which directions are blocked and by how much. OpenStreetMap is transparent and independently maintained, which matters, we are not marking our own homework on the obstructions.
3. Terrain and trees, where we can see them
Buildings are only half the story, terrain and trees block sun too. Where the Environment Agency National LiDAR Programme covers your location, the tool reads a real, laser-measured skyline that includes tree canopies and ground shape, and folds that into the shading. LiDAR coverage is expanding but not yet nationwide, currently it reaches parts of Exeter and north-west London, so on covered spots you get tree-aware figures and elsewhere the model uses buildings alone. We say which you are getting, on the result itself.
4. How much sun that is worth
Knowing when you are in sun is only useful if it maps to energy. Annual generation for an open-horizon balcony is modelled from PVGIS, the European Commission's satellite-irradiance database, for your exact location. The tool then reduces that by the shading it measured, splitting out how much diffuse skylight still reaches a partly shaded panel, to give a shading-adjusted yield range. For the saving in pounds, the postcode calculator applies your tariff on top.
What we deliberately do not claim
Accuracy also means being honest about the edges. It is a model, not a site survey, OpenStreetMap can be missing a building or estimating a height, and away from LiDAR coverage trees are not counted. A small balcony detail, a neighbour's extension, a single large tree just out of the data, can all move the real answer. So the tool is built to get you a strong, specific starting point, not to replace standing on your balcony and looking up. Every result says as much.
The part we keep to ourselves
The ingredients above are public on purpose, you should be able to see where our inputs come from. What is our own work is the join: how we stitch these sources together, how we weight buildings against LiDAR, how we build and tune the horizon so it stays fast on a phone, and how we validate it against reference data before it ships. That engineering is what makes the difference between four raw datasets and a number you can trust in a second, and it is the part we do not publish. Open inputs, our own method, is the balance we have chosen.
If you want to see it work, drop a marker on your own balcony in the shadow simulator, or go straight to pounds with the postcode calculator. Prefer sharper building detail? There is a 3D beta too.