Total Solar Resource Fraction (TSRF)
TSRF is the ratio of the solar insolation available at a surface, accounting for both shading and the tilt and orientation factor (TOF), to the insolation at the optimal tilt and orientation with no shading. Calculated as solar access multiplied by TOF, it is the single number that best describes how good a roof face is for solar.
Total Solar Resource Fraction, or TSRF, is the single figure that best answers the question "how good is this roof face for solar?" It is the ratio of the insolation a surface can actually capture, after shading and geometry are both taken into account, to the insolation an ideally tilted and oriented surface would capture with no shading at all.
It combines two separate penalties. The first is shading, measured by solar access. The second is geometry, measured by the tilt and orientation factor (TOF), which compares a roof's actual pitch and azimuth to the optimal angle for the location. TSRF is simply the product of the two:
TSRF = solar access × TOF.
That combination is what makes it useful. A steep north-facing roof with no trees around it can have 99 percent solar access and still be a poor site, because its TOF is low. A perfectly angled south roof shaded by a tall oak can also disappoint, because its solar access is low. TSRF catches both cases in one number, so you do not have to reason about shading and orientation separately when ranking roof faces.
It is widely used for qualification. Because it is auditable and location-aware, TSRF appears in incentive and certification programs as a minimum siting threshold, often around 75 percent, below which a system may not qualify for a rebate.
Why it matters for solar installers
TSRF is the number a program administrator or a skeptical customer will ask for. Reporting it per roof face lets you defend a layout, qualify for incentives, and steer the design toward the best surfaces early. solarVis computes TSRF from its hourly 3D shading analysis and per-face TOF, so the value on your proposal is traceable back to the actual model, not a rule of thumb.
Common questions
- What is a good TSRF value?
- Many quality and incentive programs treat a TSRF at or above 75 percent as a well-sited array, and some rebates set an explicit minimum in that range. Higher is better, but a lower TSRF is not automatically a deal-breaker if the economics still work; it just needs to be disclosed and modeled honestly.
- What is the difference between TSRF, TOF, and solar access?
- Solar access captures shading loss, TOF (tilt and orientation factor) captures the geometric penalty of a roof that does not face the optimal direction or angle, and TSRF combines the two by multiplying them. A roof can have perfect solar access but a low TSRF because it faces east at a shallow pitch.
- Why do incentive programs reference TSRF?
- Because it is one auditable number that certifies a system was not sited in a poor location just to claim a subsidy. Requiring a minimum TSRF protects program funds and homeowners from arrays that will chronically underproduce, which is why it appears in several rebate and certification frameworks.
