Rooftop Solar Fit Calculator

Project preset:

Start from a realistic screening case, then edit any field.

Start from:

Choose whether you know the roof-plane dimensions or only its sloped surface area.

Roof face length:

One continuous roof-plane length before edge setbacks.

Horizontal roof run:

Plan-view eave-to-ridge distance before the pitch factor.

Measured roof surface area:

Area-mode results cannot prove a real row-and-column permit layout.

Roof pitch: {{ pitchReadout }}

Select rise per 12; the current pitch remains visible in the heading.

Roof keepout profile:

Choose the closest roof condition or set a custom allowance.

Custom keepout allowance:

Edge setback:

Clear distance removed from each roof edge before fitting modules.

Panel model:

Use the exact module datasheet when comparing a quote.

Panel nameplate power:

DC watts for one module.

W DC

Panel length:

Panel width:

Panel orientation:

Roof direction:

Choose the broad roof-face direction used for the annual-energy screen.

Custom direction factor:

Peak sun hours:

Average daily equivalent full-sun hours.

h/day

System losses:

Applied after sun, direction, and tilt factors.

Panel gap:

Zero leaves the module-frame grid unchanged.

Quote or target size:

Zero omits the target-fit comparison.

kW DC

Annual electricity use:

Zero omits the annual-use offset.

kWh/yr

Installed price:

Zero omits the gross-cost line.

/ W

Rooftop solar fit takeoff
Measure	Current estimate	Basis	Copy
{{ row.label }}	{{ row.value }}	{{ row.note }}

Portrait, landscape, and target placement scenarios
Scenario	Panels	System size	Fit note	Copy
{{ row.label }}	{{ row.panels }}	{{ row.systemSize }}	{{ row.note }}

Rooftop solar layout decision checks
Check	Status	Next action	Copy
{{ row.label }}	{{ row.status }}	{{ row.note }}

A rooftop solar proposal must pass two different tests. The modules have to fit on a real roof face, and the fitted array has to produce useful energy. Gross roof area answers neither question by itself because clearances, obstructions, module dimensions, frame gaps, pitch, direction, shade, and system losses affect the result in different ways.

Physical layout is discrete. A narrow strip of remaining roof can add no capacity if another complete module will not fit. Portrait and landscape orientation may also produce different row and column counts, so a smaller module can sometimes create more total DC power than a larger high-wattage module.

Rooftop solar planning quantities
Quantity	Meaning	Common overread
Roof surface area	The sloped area of one roof face.	Assuming every square metre or square foot can hold a module.
Setback	Clear distance removed from the edges of a measured rectangle.	Treating one generic distance as the local permit layout.
Keepout allowance	A percentage reserved for obstructions and unusable layout area.	Equating it with a detailed shade or fire-access drawing.
DC system size	Fitted module count × nameplate watts.	Reading nameplate DC power as annual energy.
Annual energy	A screening estimate after sun, direction, tilt, and loss factors.	Treating a simplified yearly total as a site-specific production study.

Rectangle measurements reveal row-and-column losses. An area-only measurement can estimate a maximum count from usable area, but it cannot prove how panels land around hips, valleys, dormers, access paths, or irregular edges. Exact module and racking dimensions are essential when comparing a contractor proposal.

Energy comes after fit. Peak sun hours translate local solar resource into an equivalent daily full-sun duration; direction and roof pitch adjust that screen; system losses reduce the result for effects such as wiring, mismatch, temperature, soiling, and availability. The simplified factors are useful for comparison, not a replacement for an hourly shade and production model.

A fitted count is an early feasibility result. Structural capacity, roof condition, fire access, wind loading, electrical design, inverter limits, permits, utility interconnection, incentives, and installer verification remain separate project decisions.

How to Use This Tool:

Screen one continuous roof face at a time, starting from measurements rather than a desired system size.

Choose a project preset as a starting case, then select Roof rectangle and pitch or Measured roof surface area. Replace every preset value that does not describe the roof being reviewed.
For rectangle input, enter roof-face length, horizontal run, pitch, and edge setback. For area input, enter sloped surface area and remove unusable access or edge areas before entry because a distance setback is not applied in that mode.
Choose the closest keepout profile or set a custom percentage for vents, skylights, chimneys, shade avoidance, irregular edges, and other unusable space.
Select the exact panel model or enter nameplate watts and frame dimensions from the datasheet. Compare automatic, portrait, and landscape orientation when row placement matters.
Set roof direction, peak sun hours, and system losses for the annual-energy screen. Use a custom direction factor only when another analysis supports it.
Open Advanced for module-frame gap, quote or target size, annual electricity use, and installed price. A 0 value deliberately omits the corresponding comparison.
Review the physical panel count before DC size or annual energy. If the setback consumes the rectangle, reduce the assumed distance only after checking the measurement and applicable clearance rule.

Interpreting Results:

Panels is the selected whole-module count after the grid and area caps. DC kW is that count multiplied by module watts. If a target is short, the shortfall is reported as complete module slots rather than fractional capacity.

Annual kWh is a screening total. Direction below a 0.75 factor is marked weak, keepout at 30% or more is marked conservative, and a rectangle loses a tight-setback status only when edge clearances remove more than 35% of gross surface area. These are planning flags, not pass/fail permit rules.

Area-only input reports an area-based count without rows, columns, or a modeled edge setback.
A target fits when the physical count is at least the modules required after rounding upward.
Annual use is covered only when estimated annual kWh is at least 100% of the entered usage.
Installed cost is a simple DC watts × price-per-watt estimate and excludes financing, incentives, replacements, and operating costs.

Verify roof boundaries, setbacks, obstructions, shade, module dimensions, racking clearances, and electrical limits before using the count in a quote or permit drawing.

Technical Details:

The model has a geometric stage and an energy stage. Lengths are converted to feet and areas to square feet for the physical-fit calculation. Module counts always use whole-number floors; target module count uses an upward ceiling.

Formula Core:

In rectangle mode, pitch converts horizontal run to sloped surface width, then the edge distance is removed from both sides. The keepout allowance reduces the setback-adjusted area.

\begin{array}{lcl} p & = & \frac{\sqrt{12^{2} + r^{2}}}{12} \\ W_{surface} & = & W_{run} \times p \\ A_{setback} & = & (L - 2 s) (W_{surface} - 2 s) \\ A_{usable} & = & A_{setback} \times (1 - \frac{k}{100}) \\ N_{area cap} & = & ⌊ \frac{A_{usable}}{(L_{panel} + g) (W_{panel} + g)} ⌋ \end{array}

For each orientation, columns and rows are separately floored from the setback rectangle and module dimensions. The orientation count is the smaller of grid count and the area cap. Automatic orientation keeps portrait unless landscape fits strictly more panels. In area-only mode, the area cap becomes the count and rows, columns, orientation, and setback distance remain unproven.

DC size and annual energy use the selected whole-module count:

\begin{array}{lcl} P_{DC} & = & \frac{N \times P_{module}}{1000} \\ E_{annual} & = & P_{DC} \times H \times 365 \times F_{tilt} \times F_{direction} \times (1 - \frac{losses}{100}) \\ N_{target} & = & ⌈ \frac{P_{target} \times 1000}{P_{module}} ⌉ \end{array}

Rooftop solar screening factors
Factor	Value	Meaning
Equator-facing	1.00	Reference direction
Southeast or southwest	0.95	Broad direction screen
Mixed faces	0.90	Broad combined-roof screen
East or west	0.85	Broad direction screen
Pole-facing	0.62	Weak-face screen
Custom direction	0.45 to 1.05	User-supplied screening factor.
Tilt	0.86 to 1.00	Simplified planning factor centered at 25 degrees, reduced by 0.003 per degree of difference and floored at 0.86.

Rule Core:

Rooftop solar result boundaries
Result	Boundary	Interpretation
Target fits	Fitted panels ≥ required target panels	Physical slot count reaches the entered DC target.
Use covered	Annual offset ≥ 100%	Screened annual energy reaches the entered annual use.
Tight setback	Setback removes > 35% of rectangle surface area	Verify the roof boundary and applicable clearances.
Conservative keepout	Keepout ≥ 30%	A large share of setback-adjusted area is withheld.
Weak face	Direction factor < 0.75	Direction materially reduces the screening yield.

With the default 14.02 m roof-face length, 5.49 m horizontal run, 6/12 pitch, 0.91 m setback, 14% keepout, and 400 W modules measuring 1.72 m × 1.13 m, portrait fits 20 modules and landscape fits 21. Automatic selection therefore returns 21 modules, 8.4 kW DC, and about 11,810 kWh/year under 4.5 peak sun hours, equator-facing direction, and 14% losses.

Accuracy and Safety Notes:

The physical count and annual energy are screening estimates, not construction documents or a bankable production forecast.

Confirm local setbacks, fire access, wind and snow loading, roof structure, roof condition, and permit requirements.
Use module and racking datasheets for exact dimensions, gaps, clamps, attachment zones, and electrical limits.
Area-only results cannot prove a real row-and-column layout or apply a distance setback.
The direction and tilt factors are simplified planning assumptions, not PVWatts calculations.
Use a site-specific shade and production model before evaluating savings, financing, or guaranteed output.

References:

Solar Photovoltaic System Design Basics, U.S. Department of Energy.
Permitting and Inspection for Rooftop Solar, U.S. Department of Energy.
PVWatts Calculator Version 8, National Renewable Energy Laboratory.
PVWatts Version 5 Manual, National Renewable Energy Laboratory, September 2014.