Photovoltaic Potential: Is My Roof Sunny Enough?

Whether a photovoltaic system is worthwhile depends less on gut feeling, such as "we get a lot of sun," and more on measurable factors: roof orientation, pitch, shading from buildings or terrain, and whether the sun reaches the roof unobstructed during the most productive parts of the day. This article shows home buyers and owners how to assess their roof's PV potential realistically and how sunlight-hour and shading visualizations help compare locations objectively.

Why "Sunny" Is Not Enough: What Photovoltaics Really Need

Many properties seem bright during a viewing, yet a photovoltaic system may later deliver less than expected. The reason is that for PV it is not enough that it gets bright at some point in the day. What matters is whether direct sun reaches the roof surfaces during the productive time windows and whether that solar input remains stable throughout the year. The most important drivers are surprisingly geometric: where the sun sits in the sky in terms of height and angle, how the roof is oriented, whether south or north, east or west, how steep it is, and which obstacles cut across the sun path, such as neighboring buildings, the terrain horizon, or mountains. Especially in dense development, the sun may generally shine, but still disappear behind building edges for weeks in the morning or afternoon. Another point is often underestimated: shading is not just "less light." It can reduce PV yield disproportionately if partial roof areas, for example because of a chimney, dormer, roof structures, or a neighboring roof edge, regularly receive shade. For any location decision, the key is therefore not just to see shading patterns, but to place them in time.

• ✓ Do not ask only "How bright is it?" Ask "When does direct sun reach the roof, and for how long?"
• ✓ Assess shading in time terms: shade around midday usually matters more for PV than a short shadow at the edge of the day.

Orientation and Pitch: Why South Is Not Automatically Perfect

A common rule of thumb is that a roof surface facing south often provides the most stable conditions for direct sun throughout the day. But south is not a free pass. What matters is whether the sun actually sits above the obstacles over the course of the year, and whether the roof does not fall into structural shade during critical months, especially in winter. East- and west-facing roofs can make a lot of sense if they fit your electricity-use profile: east tends to deliver in the morning, west more in the afternoon and evening. For households that are not home much during the day, west can even feel more practical. At the same time, if surrounding buildings or a high terrain horizon block exactly those time windows, that orientation loses its advantage. The pitch of the roof acts like an amplifier or dampener depending on the sun's position. In summer the sun is high, in winter it is low. A roof that works very well in summer may benefit much less in winter, and winter also brings longer shadows that are especially likely to come from neighboring buildings.

• ✓ Always assess orientation together with obstacles: south helps little if the winter sun path is blocked.
• ✓ Use east and west deliberately: yield is distributed differently across the day and may fit your electricity demand better.

Shading Map as a Starting Point: How to Read the Heatmap Correctly (Blue to White)

In the Relocheck report, the shading map is explained as a heatmap. It visualizes daylight hours and shading patterns with a color gradient from blue to white, with white marking the highest amount of light. This view is extremely helpful for seeing at a glance whether the surroundings are fundamentally strong in light or whether persistent shadow zones dominate. For PV, this map indicator is useful because large, continuous bright areas suggest that obstacles such as surrounding buildings or terrain limit incoming sunlight less strongly. Darker areas point to recurring shading, which is exactly where the classic PV problems tend to arise: lower yield during important hours, stronger seasonal differences, and greater dependence on weather. A spatially detailed view is essential. If a property lies on an edge between bright and dark areas, the difference in practice can be huge, for example between the street-facing and courtyard-facing side, or between the front roof plane and a rear roof section. For PV, that means not just looking at the house in general, but specifically at the usable roof surfaces, such as the south-facing area, a dormer, flat roof sections, or a carport.

• ✓ Read the heatmap as a pattern: broadly bright means robust potential, broadly dark means structural shading risk.
• ✓ Pay special attention to transitions: there the exact roof surface, front or rear, dormer, or roof structures, becomes decisive.

Monthly Maps: The PV Reality Check Across Summer and Winter

The report explains why monthly maps matter: the angle and height of the sun change throughout the year, which means the amount of daylight changes from month to month, and separate maps are created for each month. For photovoltaics, this comparison is extremely valuable because PV yield is strongly linked to the sun's position and the duration of direct solar exposure. Many roofs perform excellently in June and July because the sun is high and the days are long. The very same roofs can be much more heavily shaded in December and January because the sun is low and shadows from buildings or terrain stretch much farther across the roof surfaces. To use monthly maps in practice, compare at least one summer month and one winter month. Then check whether the roof surfaces stay in brighter areas in both months or whether they shift into much darker patterns in winter. A strong winter drop is not automatically a dealbreaker, but it changes expectations. You can then plan more realistically in terms of system sizing, profitability, storage, and autonomy targets, and avoid disappointment. Another advantage is that monthly maps also help you mentally simulate the effect of future densification. If the existing development already creates critical winter shadows, additional buildings nearby can make the situation worse, exactly where it is already tight.

• ✓ Always check contrast months, summer plus winter, otherwise seasonal shading is easily overlooked.
• ✓ Interpret winter decline correctly: not automatically bad, but it clearly affects yield expectations and planning.

Directional Table: Making Orientation Measurable

To make the monthly maps more concrete, the report explains that daylight hours are additionally broken down into a table by cardinal direction, north, south, east, and west. For PV, this is especially helpful when comparing multiple properties. Two houses may be similarly priced, yet roof orientation and the distribution of light hours can be completely different. With the table, you can see more quickly whether an orientation is solid year-round or only really works in certain months. Practical reading rules are simple. If south, or south-west, shows high light-hour values over many months, that indicates more stable conditions. If east or west shows strong time windows, that can make sense depending on your use profile, but only if the sunlight hours do not collapse because shading cuts off the typical usage periods. If north drops sharply in winter, that is expected. The important question is whether other roof surfaces, such as south or west, compensate for it.

• ✓ Use the table for location comparison: the same measurement logic across several houses instead of relying on viewing impressions.
• ✓ Think in time windows: which cardinal direction delivers when you need or want to feed electricity into the grid?

Sun-Path Horizon View: The Most Precise Sign of Shading from Buildings or Terrain

When it comes to PV planning, the question "Does the sun actually rise above the obstacle?" is often more important than any general statement. That is exactly where the sun-position analysis in horizon view helps. The report describes the terrain horizon in light gray and neighboring buildings in dark gray. If the sun path lies above the buildings, direct solar input is possible and shown as a continuous line. If terrain or buildings block the sun, the path lies below them and is shown as a dotted line. This logic translates directly to PV. Dotted paths in months with a low sun angle are a warning signal that the roof surfaces receive structurally less direct sun during exactly the critical windows, in the morning and afternoon and, in winter, often also during the already short daylight period. For buyers, this is especially valuable because it immunizes you against viewing bias. Even if you stand there on a sunny day, the graphic shows whether that is generally true or whether the sun just happens to be above the edge at that moment.

• ✓ Understand solid versus dotted as a clear rule: possible versus blocked, especially check the winter months.
• ✓ Identify the type of obstacle: building in dark gray versus terrain in light gray implies different levels of changeability.

Trees, Cloud Cover, Reality: What Models Do Not Capture and How to Still Decide Safely

A sound PV check also requires a look at model limits. The report explicitly states that shading maps do not account for variables such as cloud cover and tree cover, even though both can influence the actual amount of sunlight received. For photovoltaics, that means the visualizations provide a very strong structural statement about buildings, terrain, and seasonal shading geometry, but you still need to account separately for vegetation and weather. Trees are especially tricky because their effect varies greatly depending on species, crown density, and season. A tree can create shading in summer and reduce yield, while blocking less in winter if it is deciduous, whereas evergreen crowns shade more strongly year-round. A practical safeguard is to combine the map logic with an on-site look toward the south, east, or west, depending on the relevant roof surfaces, and to watch for tall trees, planned new buildings, and roof structures. Especially in purchase decisions, it makes sense to weight worst-case months, meaning winter, more heavily because the sun is low anyway and extra obstacles matter even more.

• ✓ Check tree cover separately: height, distance, crown density, and cardinal direction, especially south and west.
• ✓ Do not confuse cloud cover with shading: the maps explain geometry, weather explains day-to-day variability.

Public Solar Cadastres as a Supplement: How to Get a Second, More Yield-Oriented Assessment

Sunlight-hour and shading maps explain very well whether direct sun is geometrically possible and how stable the light remains across months. For PV yield, public solar cadastres often add an energy perspective, typically expressed as annual potential in kilowatt-hours per square meter per year or as suitability classes. Many regions in Austria provide such tools. For example, the City of Vienna offers a solar potential cadastre and explains there that it can quickly show whether and how well a roof surface is suited to solar energy. There are also explanations that classify suitability by annual solar-energy potential, for example very good, good, or medium. The province of Salzburg likewise points out that building solar potential can be accessed via the geodata viewer SAGISOnline and gives a rule of thumb for the annual irradiation level above which roof areas can generally be considered suitable. The best approach is a two-system method: first assess the geometry, meaning shading, sun paths, and monthly logic, then plausibility-check the energy side through a solar cadastre. If both perspectives look good, the probability is high that the roof is not only theoretically bright, but practically suitable for solar use.

• ✓ Geometry first, energy second: first understand shading, then check yield plausibility through a solar cadastre.
• ✓ If the solar cadastre looks good but shading is critical, check the timing windows, winter and morning or evening, and set realistic expectations.

Decision Support for Buyers: Understand PV Potential as Both a Location and Investment Factor

For home buyers, PV potential is more than a technical topic. It is a location and use topic. A roof that receives stable direct sun throughout the year and has little structural shading gives you more options: lower ongoing energy costs, easier retrofit planning, and often greater attractiveness at resale. It is important not to view PV in isolation. Shading often comes from the same factors that also affect residential quality, such as dense development, tall neighboring buildings, valley locations, or a strong terrain horizon. Anyone who recognizes these patterns early makes not only a better PV decision, but usually a more robust property decision overall. If you are comparing several houses, a standardized, data-based view helps: identical visualizations, identical logic, and a clear month-by-month comparison. That turns PV potential from a guess into a traceable property that you can integrate into your purchase decision.

• ✓ See PV potential as a location feature: structural shading is rarely something you can simply plan away.
• ✓ Always compare multiple properties in a standardized way: same months, same roof surfaces, same cardinal directions.