Solar Window

Key Takeaways

• Solar window = usable solar time at a site.
• Affected by horizon obstructions and seasonal sun-path.
• Quantified via TSRF in shading analysis.
• 9 AM - 3 PM solar window commonly cited for residential rooftop optimum.

How the Solar Window Is Determined

A site’s solar window is not read off a single sunrise-to-sunset chart. It is built by combining the site’s sun-path (where the sun sits in the sky, hour by hour, month by month) with a horizon profile of everything around the array that could block low-angle light, a neighboring roofline, a tree line, a hill, or an adjacent structure.

Wherever the sun’s position sits above that horizon obstruction line, the hour counts toward the usable window; wherever it sits below, that hour is lost to shading regardless of how sunny the day is elsewhere. This is the same underlying logic that a shading analysis formalizes into a quantitative figure, the Total Solar Resource Fraction, or TSRF, and that a horizon profile exists specifically to capture in the field.

Worked Example

Consider a rooftop with a clear view of the sky except for a row of trees to the east and a neighboring two-story building to the west. In summer, when the sun rises far to the northeast and sets far to the northwest, both obstructions clip only the very first and last few minutes of daylight, so the solar window stretches close to the full day.

In winter, the sun’s arc is lower and shifted toward the south, so the same tree line and building intrude on a much larger share of the morning and afternoon, the solar window can shrink to something closer to the mid-morning-to-mid-afternoon core, which is why designers report TSRF as a monthly profile rather than a single number.

A Solar Pathfinder reading taken on-site, or the equivalent obstruction overlay inside PVsyst’s near-shading module, is what turns this qualitative picture into the numbers used in a bankable yield estimate.

Related Reading

Solar window sits at the intersection of shading and yield modeling, so it shows up wherever a designer has to prove a site’s energy output to a lender or client.

For the software side of quantifying it, see SurgePV’s generation and financial modeling tool, which turns a site’s usable solar window into a bankable energy and cash-flow forecast, and our own breakdown of solar shading analysis software for how TSRF and horizon obstructions get modeled across a full 8,760-hour year.

If you’re building the obstruction geometry yourself in PVsyst, the 3D near shading scene builder tutorial walks through constructing the horizon and near-shade objects that ultimately define a site’s solar window.

Because the solar window is only as good as the irradiance data behind it, pairing it with state-wise resource data, such as Qbits Energy’s state-wise solar irradiance reference, helps confirm whether a shortened window at a given latitude still supports the expected annual yield.

Disclosure: Qbits Energy and SurgePV are Heaven Designs group companies.