Solar Access
Solar access is the percentage of available solar radiation that reaches a surface once shading is accounted for, calculated as the irradiance received with shading divided by the irradiance that would be received with no shading. A value of 96 percent means 4 percent of the potential sunlight is lost to surrounding trees, buildings, and other obstructions.
Solar access is the share of the available sunlight that actually reaches a surface after every obstruction has been accounted for. It is expressed as a percentage and calculated as the irradiance received under real, shaded conditions divided by the irradiance the same surface would receive with a clear sky and no obstructions.
The intuition is simple. If a roof plane would receive 1,000 kWh/m² per year with nothing around it, but nearby trees and a chimney cut that to 960 kWh/m², the solar access is 96 percent. The missing 4 percent is the shading loss. Because it isolates the obstruction penalty from every other factor, solar access is the cleanest way to compare one roof face, or even one panel, against another.
It is a per-point quantity. Solar access varies across a single roof, so good tooling reports it panel by panel rather than as one number for the whole array. A module tucked behind a dormer can sit at 70 percent while a module three rows up sits at 98 percent, and that difference drives both layout and, once panels are wired in series, string-level losses.
It feeds directly into yield. Solar access is one of the inputs that turns a theoretical production figure into a realistic one. Combined with the tilt and orientation factor it becomes the Total Solar Resource Fraction, the single number that best summarizes site quality.
Why it matters for solar installers
Solar access is where a design either earns or loses the customer's trust. If you place panels on a roof face at 75 percent solar access without flagging it, the system underperforms and the payback slips. solarVis reports solar access per panel inside the 3D design canvas, so you can move modules off shaded zones before you simulate, and show the customer exactly why one part of the roof was left clear.
Common questions
- What is a good solar access value?
- For a productive rooftop, look for annual solar access above roughly 90 percent. Values in the mid to high 90s indicate a nearly unobstructed roof. Anything below about 80 percent signals meaningful shading that will pull down yield and should be investigated obstruction by obstruction before you commit a layout.
- How is solar access different from TSRF?
- Solar access measures only the shading loss, how much sunlight is blocked by obstructions. TSRF (Total Solar Resource Fraction) multiplies solar access by the tilt and orientation factor, so it also folds in the geometric penalty of a roof that does not face the optimal direction or angle. Solar access answers "how shaded", TSRF answers "how good overall".
- How is solar access measured?
- Modern 3D design tools cast the sun path across a full year against a model of the roof and its surroundings, then compare the shaded irradiance at each point to the unshaded case. The result is a per-point or per-panel percentage. Older handheld tools estimated it from a fisheye horizon image instead.
