What a Shadow Horizon Is, and Why It Decides Your Solar Yield

The sky is a dome, and your obstacles eat into it

Stand on the spot where you want to put solar panels and look up. The sky is a dome. The sun travels across that dome on a curved path that shifts with the seasons. Every tree, roof ridge, chimney and neighbouring building rises up from the ground and covers part of the lower dome. That outline, the height of everything blocking your view of the sky measured all the way around you, is the shadow horizon. Engineers also call it the horizon profile.

To describe it precisely you need two angles. The first is the compass bearing, called the azimuth. By convention 0 degrees is north, 90 degrees is east, 180 degrees is south and 270 degrees is west. The second is how high an object reaches above the flat horizon, called the elevation or altitude angle, from 0 degrees (flat, at eye level) up to 90 degrees (straight overhead).

A shadow horizon is just a table of those two numbers together. For each compass bearing, what is the elevation angle of the highest thing blocking the sky? A line of poplar trees due south might reach 25 degrees. The open field to the west might be 2 degrees. A neighbour's house to the southeast might be 18 degrees. Plot those points and join them, and you get a jagged line that separates open sky above from blocked sky below. Everything below the line is shade. Everything above it is potential sunlight.

Why the line matters most in winter

A shadow horizon would not matter if the sun were always high overhead. It is not. The sun's height changes enormously through the year, and that is the whole reason this measurement decides your yield.

At solar noon the sun's elevation equals 90 degrees minus the difference between your latitude and the sun's declination. The declination swings between plus 23.45 degrees at the June solstice and minus 23.45 degrees at the December solstice. For a site at 50 degrees North, roughly Frankfurt or Prague, that gives a midday sun about 63 degrees high in June and only about 16 degrees high in December. That is a swing of nearly 47 degrees between the seasons.

Now the danger is obvious. In June the sun rides high across the sky and clears most obstacles easily. In December it barely lifts off the horizon, skimming low across the south for a few hours before setting again. The exact band of sky where the winter sun lives, low and toward the south, is precisely the band that trees and buildings are most likely to block. A row of conifers that casts no shadow on your panels in summer can sit squarely in front of the December sun.

This is why a roof can look perfect on a sunny afternoon in July and still underperform. The summer test passes. The winter test, the one that matters for a year-round system, quietly fails. Only a measured shadow horizon shows you both at once.

Where on the horizon the damage is worst

Not all obstructions are equal. An obstacle due north of a site in the northern hemisphere blocks almost nothing, because the sun is never in the northern sky during the productive hours. The sun rises in the southeast, climbs through the south and sets in the southwest. So the bearings that matter are roughly southeast (about 135 degrees) through south (180 degrees) to southwest (about 225 degrees).

A tree or building in that southern arc removes the sun during the middle of the day, when it is highest and strongest and delivering the most energy per minute. The same object placed due east only costs you some early morning light, which is weaker and shorter. This is why two roofs with the same total amount of greenery around them can have very different yields. Position on the horizon is everything.

There is a simple rule of thumb. Walk the south side of the spot first. If something there reaches more than about 20 degrees of elevation, expect a real winter loss. If the southern horizon is clean below 10 degrees, the site is probably good, and you can worry less about clutter to the north and far east or west.

• South arc (135 to 225 degrees): every degree of obstruction here costs the most, especially in winter.
• East and west (90 and 270 degrees): obstructions cost morning or evening light, a smaller and more seasonal loss.
• North (0 degrees): almost irrelevant for panels in the EU, since the sun never shines from there during useful hours.

How professionals have measured it

Solar installers have measured the shadow horizon for years, because they know it drives yield. Two tools dominate the trade.

The Solar Pathfinder is the low-tech classic. It is a polished transparent dome placed over a printed sun-path diagram for your latitude. Trees and buildings around you appear as reflections on the dome, and wherever a reflection crosses a sun-path arc, that hour is shaded. Because it works by reflection rather than waiting for real shadows, it can be used at any time of day in any weather. It costs roughly 300 to 350 USD, but reading it accurately takes practice and the result is only as good as the operator.

The Solmetric SunEye is the digital version. It uses a fish-eye camera plus an electronic compass and inclinometer to capture a 360 degree panorama, then automatically traces the skyline and overlays the sun paths to compute a solar-access percentage. It is fast and precise, and it exports a horizon profile and PDF report. It is also a professional instrument: the SunEye 210 lists around 2,195 USD. That price makes sense for a busy installer and no sense at all for a homeowner checking one balcony.

Both tools prove the point: the shadow horizon is worth measuring properly. The barrier has always been cost and skill, not value.

Feeding the horizon into a yield calculation

Measuring the skyline is only half the job. The other half is turning it into a kilowatt-hour number, and the free standard tool for that in Europe is PVGIS, run by the European Commission's Joint Research Centre.

PVGIS already includes a horizon by default. It reads terrain from a digital elevation model at about 90 metre resolution (3 arc-second data) and works out how distant hills and mountains shade each site. That captures the lay of the land. What it cannot see is the tree in your garden or the house next door, because those are too small and too local for a 90 metre grid.

This is where your own measurement comes in. PVGIS lets you upload a custom horizon file. The format is plain and strict: a simple text or CSV file with one elevation value per line, given in degrees, going clockwise starting from north. The values are assumed to be evenly spaced around the full circle, so 48 numbers means one reading every 7.5 degrees of bearing. Upload your measured shadow horizon and PVGIS recomputes the yield with your actual obstructions included, month by month, and shows the sun elevation at the June and December solstices so you can see the winter problem directly. Suddenly the estimate reflects your spot, not an idealised open field.

Solargis, PVsyst and other professional packages accept horizon profiles the same way. The measurement is portable. Capture it once and you can feed it into whichever calculator you trust.

Capturing it on a phone, for free

The maths and the calculators have been free and public for years. The missing piece was an affordable way to measure the skyline on site, without a 2,000 USD instrument or a dome that takes training to read.

A modern phone already carries the right sensors: a camera, a compass and motion sensors that track exactly where the phone is pointing. Augmented reality combines them. You stand on the precise spot, the roof tile, the balcony rail, the patch of garden, and sweep the phone across the sky. The app records the elevation angle of the skyline at each bearing as you go, building the same horizon profile that a SunEye would, from hardware you already own.

This is what SunMeasure does. It captures the shadow horizon on the exact spot, then combines it with location irradiance data from official sources such as PVGIS, the Global Solar Atlas, Copernicus CAMS, DWD and NASA POWER to estimate the realistic yield and suggest the best tilt and orientation. The point is not the app, though. The point is the concept. Map and satellite tools like Project Sunroof or a Solarkataster model the roof but cannot see the neighbour's spruce or the chimney three metres away. The shadow horizon is the one input that only an on-site measurement captures, and it is often the difference between a good estimate and a wrong one.

Learn to read the line between open sky and blocked sky, pay closest attention to the south, and remember that the winter sun sits low. Do that, and you understand the single geometric fact that decides whether a spot is worth wiring up.