Is Your Roof Good for Solar? A 5-Minute Self-Check

The short answer, and the six things that decide it

Most pitched roofs in Germany are good enough for solar. The question is not usually yes or no. It is how good, and whether the yield justifies the cost at your specific address.

Six factors decide it. Orientation (which way the roof faces). Tilt (how steep it is). Usable area (how many panels fit around chimneys, windows and vents). Shading (trees, neighbouring buildings, your own chimney). Roof age and structure (whether it can carry panels for 25 years). And your region's irradiance (how much sun your part of the country actually gets).

Five of these you can estimate from your sofa in a few minutes. The sixth, shading, is the one that map and satellite tools get wrong, and it is often the factor that makes or breaks a spot. We will come back to it.

South is best, but east-west is now fine

Orientation, sometimes called azimuth, is the compass direction your roof slope faces. South is the textbook best because the panels face the sun at midday when it is strongest.

The penalty for not facing south is smaller than most people think. According to Fraunhofer ISE, the loss from an east or west orientation compared with south is typically only 5 to 10 percent, as long as the tilt is reasonable. That is why east-west roofs have become common in Germany. Fraunhofer data on registered systems shows south orientation falling from 61 percent of installations in 2000 to 42 percent by 2019, while east, west and combined east-west setups grew steadily.

An east-west roof also spreads production across morning and evening rather than concentrating it at noon, which often matches household use better. The orientation that does hurt is north. A north-facing slope can yield up to about 50 percent less than south, and in most cases is not worth covering.

To check your own roof, open a map app, find your house, and note which way the main slope points. Anything from southeast through south to southwest is excellent. Pure east or west is still clearly worthwhile.

Most roofs are already at a workable tilt

Tilt is the angle of the roof from horizontal. For Germany, the sweet spot for annual yield is roughly 30 to 35 degrees facing south, with steeper angles slightly favoured in the north and flatter ones in the south. Most German pitched roofs sit between about 30 and 45 degrees, so they are already in or near the ideal band. You do not need to hit the exact optimum. A few degrees off costs only a small percentage.

Flat roofs are fine too. Panels go on tilted frames set to a chosen angle, though rows must be spaced so they do not shade each other.

For area, a rough rule is that a modern panel takes about 1.7 to 2 square metres and produces somewhere near 400 to 450 watts. So 10 panels need roughly 18 to 20 square metres of clear roof and give about 4 kilowatts peak. Subtract space around chimneys, skylights, vents and dormers. What matters is the largest unbroken rectangle of roof, not the total footprint.

The yield numbers for Germany

Germany receives roughly 950 to 1,220 kWh per square metre of global irradiation per year, measured by the DWD and reported by Fraunhofer ISE, with a long-term national average near 1,100 kWh per square metre. The south, around Bavaria and Baden-Wuerttemberg, gets the most. The north, around Hamburg and Schleswig-Holstein, gets the least.

Translated into output, a well-oriented system produces about 900 to 1,100 kWh per kilowatt peak per year in central Germany, rising to 1,000 to 1,200 in the sunny south. The gap between Hamburg and Munich is real but modest, roughly 16 percent. Solar is economic across the whole country, not just the south. Your region nudges the result. It rarely decides it on its own.

About three quarters of the annual yield arrives in the summer half year, April to September. A solar roof is a yearly average, not a winter power station.

The factor map tools cannot see

Shade is where good roofs quietly turn into mediocre ones, and it is the hardest thing to judge from a map. Panels are wired in series, so the weakest one drags down the rest. A single shaded cell can cut a string's output by a third or more. A fully shaded panel in a ten-panel string can cost most of that string's production. Bypass diodes and per-panel optimisers reduce the damage but do not erase it.

The trouble is that the things casting shade are exactly what satellite tools miss. A neighbour's tree that has grown two metres. A chimney on the south side. A telephone pole. A church spire to the southwest. Tools like Google Project Sunroof, the regional Solarkataster and SunCalc model the sun's path well, but they work from aerial imagery and rough 3D and routinely underestimate real obstructions at ground and roof level.

The honest way to settle it is to measure the shade from the actual spot. This is what a shadow horizon is: the altitude, in degrees above the horizon, of everything blocking the sky around you. This is the gap SunMeasure is built for. You stand on the roof or balcony, sweep your phone across the sky, and it records the real obstruction line, then combines it with official irradiance data to show how much sun the spot loses and when. It measures conditions instead of guessing from a satellite, which removes the single biggest source of error in a roof assessment.

When a roof is not worth it

Solar is not right for every roof, and a good assessment says so. Skip or delay if your situation matches one of these.

A roof near the end of its life. Panels last about 25 years. If the covering needs replacing within five to ten years, renew the roof first, or you will pay to remove and refit the array. Roof age and structural condition should be checked before anything else, ideally by a roofer.

A north-only slope with no usable alternative. The up-to-50-percent loss rarely pays unless you have nothing else and a strong reason.

Heavy, unavoidable shade. A protected tree to the south, a tall building close by, or a deep dormer pattern can remove enough midday sun that the economics stop working. Measure it before deciding, because shade is the factor most often misjudged in both directions.

• Roof covering due for replacement within roughly 5 to 10 years: fix the roof first
• Only a north-facing slope available: yield up to about 50 percent lower than south
• Permanent, heavy shading across the midday hours that cannot be removed
• A structure that cannot safely carry roughly 15 to 25 kg per square metre for 25 years

Your 5-minute self-check

Run these five steps now. Together they give you a solid first read, and they tell you whether a closer look is worth it.

If the first four come back positive and the only open question is shading, you already know where the real uncertainty lives, and that it is the one worth measuring on site rather than guessing from above.

• 1. Direction: open a map, find your main roof slope. South to east or west is good. North alone is usually out.
• 2. Tilt: estimate the pitch. Most German pitched roofs at 30 to 45 degrees are already near ideal.
• 3. Area: picture the largest clear rectangle, minus chimneys and skylights. Each panel needs about 1.7 to 2 square metres.
• 4. Roof age: how many years of life does the covering have left? Under 10 means deal with the roof first.
• 5. Shade: stand on the spot and look south, east and west. Note every tree, building and chimney that breaks the skyline, then measure their height in degrees rather than guessing.