Estimate how many solar panels fit on your roof and the optimal layout. Free solar panel layout estimator for designers, installers, and homeowners.
Before you can design string configurations, size an inverter, or submit a permit application, you need to know one fundamental number: how many solar panels fit on the roof? That number determines system size, inverter selection, production estimates, and project economics. Getting it wrong wastes time on proposals that don't survive a site assessment — or worse, designs that fail the permit review.
This solar panel layout estimator calculates the maximum panel count for a rectangular roof section based on your actual roof dimensions, panel specifications, row and column spacing, and required setbacks. It returns panels per row, number of rows, total panel count, array footprint dimensions, total array area, estimated system size in kW, and estimated annual production in kWh. All in seconds, with no CAD software required.
Use it in the field on a tablet to quickly assess a roof's potential before climbing down. Use it in the office to sanity-check a layout before investing hours in Aurora or Helioscope. Use it when talking to a homeowner to instantly show how many panels their specific roof supports — before they've agreed to a single site visit.
Subtracts fire code setbacks from all roof edges before calculating usable area, giving you a compliant panel count rather than an optimistic number that won't survive AHJ review.
Enter any panel width and height in inches to match your chosen module exactly — from compact 390W residential panels to large-format 700W commercial bifacial modules. Works for both portrait and landscape orientation.
Converts panel count directly to system kW and annual kWh production using your panel wattage and location's peak sun hours — so you immediately know the project's energy and financial potential.
Measure roof dimensions from the ground or ladder and enter them into this tool on your phone or tablet immediately. Before you even drive away, you can tell the homeowner exactly how many panels fit, what system size the roof supports, and how that translates to annual production — turning a brief site visit into a compelling first proposal discussion.
Use the array dimension outputs to verify that your layout drawing accurately represents the physical array footprint. The tool confirms that the setback-compliant layout fits within the roof boundary, which is exactly what the AHJ needs to see on your site plan. Array dimensions from this tool can be entered directly into your CAD permit drawing.
Run the calculator with two different module sizes — for example, 400W panels versus 430W panels — to see which delivers more total system kW on the same roof section. A larger panel reduces panel count but each panel delivers more watts; sometimes fewer larger panels fit in the same space and produce more total energy than more smaller panels.
Measure Roof Width & Height
Input the width (horizontal dimension parallel to the eave) and height (dimension from eave to ridge) of the roof section you plan to use for panels, in feet. For complex roof shapes, break the area into rectangular sections and run the calculator separately for each. Use a tape measure in the field or measure from Google Earth/Nearmap satellite imagery in the office.
Enter Panel Dimensions
Input your chosen panel's width and height in inches. For portrait orientation (most common for residential), the panel height is the longer dimension (typically 65–68 inches) and the width is the shorter dimension (typically 39–42 inches). For landscape orientation, simply swap width and height. Check your panel datasheet for exact module dimensions — do not use rounded figures.
Set Row & Column Spacing
Row spacing (vertical gap between panel rows) is typically 4–8 inches for flush-mount roof installations. This gap allows thermal expansion, racking cross-bar access, and minimal inter-row shading for low-pitch roofs. Column spacing (horizontal gap between panels in the same row) is typically 0–2 inches — just enough clearance for module junction boxes and wiring management.
Enter Setback Distance
Input the required setback from all roof edges in feet. Most residential installations require a 3-foot setback from the ridge, rakes (sloped edges), and eave for firefighter access paths per IFC 605.11. Some local jurisdictions have modified requirements — check with your AHJ before finalizing. A 36-inch (3 ft) setback is the most common default for residential projects in the US.
Enter Panel Wattage & Peak Sun Hours
Input your panel's nameplate wattage (Wp) and your location's daily peak sun hours (PSH). These two inputs let the calculator convert panel count directly to system kW and estimated annual kWh production, so you immediately know the project's energy potential alongside the physical layout numbers.
Review All Seven Outputs
Instantly see: panels per row, number of rows, total panel count, array width (ft), array height (ft), total array area (sq ft), estimated system size (kW), and estimated annual production (kWh). If total panel count is constrained by roof area, the tool highlights this so you can adjust panel size, spacing, or setback to optimize the layout.
Output 1
Panels Per Row
The number of panels that fit horizontally across the usable roof width, after setbacks have been subtracted. Calculated as floor((Usable Width × 12) ÷ (Panel Width + Column Spacing)). This determines the horizontal extent of each string and is a key input for string inverter sizing — each row may correspond to one or two strings depending on voltage requirements.
Output 2
Number of Rows
The number of horizontal panel rows that fit vertically on the roof from eave to ridge, after setbacks are subtracted. Calculated as floor((Usable Height × 12) ÷ (Panel Height + Row Spacing)). Multiple rows stacked vertically may require consideration of inter-row shading if the roof pitch is very low and rows are closely spaced.
Output 3
Total Panels
Panels per row multiplied by number of rows. This is the maximum panel count for the specified roof section under the given setback and spacing constraints. This number directly determines system size. For multi-section roofs, run each section separately and sum the totals while considering string inverter configuration constraints.
Output 4
Array Dimensions (ft × ft)
The actual footprint of the panel array on the roof surface, in feet. Array Width = (Panels per Row × Panel Width) + ((Panels per Row − 1) × Column Spacing), all divided by 12. Array Height = (Rows × Panel Height) + ((Rows − 1) × Row Spacing), all divided by 12. Use these dimensions to verify the array fits within the usable roof area and for placement in your permit drawing.
Output 5
Total Array Area (sq ft)
Array Width multiplied by Array Height. Compare this against your usable roof area to confirm the array fits with margin to spare. Note that array area is smaller than usable roof area because the floor() function discards fractional panels. The remaining space after the array is unused roof area at the edges within the setback-compliant zone.
Outputs 6 & 7
System Size (kW) & Annual Production (kWh)
System size = Total Panels × Panel Wattage ÷ 1,000. Annual Production = System kW × Peak Sun Hours × 365 × 0.80 (system efficiency). These two outputs bridge the physical layout to the project's energy and financial case — connecting roof constraints directly to the system's production potential and economics.
This tool uses a simple but precise geometric packing algorithm based on industry-standard roof layout methodology. All inputs are in the standard units used by solar installers and permit designers. The calculation assumes a single rectangular roof section; for multiple sections or L-shaped roofs, run the calculator separately for each rectangular segment.
IFC Section 605.11 requires 36-inch minimum access pathways. Ridge setback is also typically 36 inches. Some AHJs allow reduced setbacks for specific roof configurations — always confirm locally.
FLOOR() ensures only complete panels are counted — no fractional panels. This is the correct approach for layout planning, as partial panels cannot be physically installed.
Standards reference: IFC 2021 Section 605.11 (solar energy system fire access pathway requirements); NREL PVWatts v8 (production calculation methodology); NEC 690 Article (photovoltaic system design).
Calculations sourced from SurgePV’s Solar Panel Layout Estimator — surgepv.com/tools/solar-panel-layout-estimator/
Representative specifications for popular residential and commercial solar modules in 2026. Dimensions are typical — always verify with the manufacturer's current datasheet before finalizing a layout. All measurements are for portrait orientation (height = long dimension).
| Panel Type | Wattage | Width (in) | Height (in) | Area (sq ft) | W/sq ft | Typical Use |
|---|---|---|---|---|---|---|
| Residential Mono 60-cell | 390W | 39.4" | 65.9" | 18.0 | 21.7 | Tight residential rooftops |
| Residential Mono 66-cell | 415W | 40.0" | 66.9" | 18.6 | 22.3 | Standard residential, most common |
| Residential Mono 72-cell | 430W | 41.1" | 68.5" | 19.6 | 22.0 | Higher output residential |
| High-Density Mono | 445W | 41.8" | 68.9" | 20.0 | 22.2 | Space-constrained premium installs |
| Commercial Bifacial 72-cell | 480W | 44.0" | 85.0" | 26.0 | 18.5 | Commercial rooftop, carports |
| Commercial Bifacial 78-cell | 530W | 44.5" | 90.2" | 27.8 | 19.1 | Large commercial ground-mount |
| Large-Format Commercial | 700W | 52.0" | 103.0" | 37.2 | 18.8 | Utility-scale, large commercial |
| Building-Integrated (BIPV) | 300W | 39.5" | 65.0" | 17.8 | 16.8 | Architectural integration, slim profile |
This tool calculates a theoretical maximum for a clear rectangular roof section. Real roofs have skylights, vents, exhaust fans, HVAC units, chimneys, and dormer windows — each of which requires clearance that eats into your panel count. Always walk the roof (or review high-resolution aerial imagery) before finalizing a layout based on this tool. Deduct obstruction footprints from your usable roof dimensions before entering them.
Using a rounded "about 66 by 40 inches" for panel size instead of the exact datasheet value of 66.9 by 40.0 inches sounds trivial — but across 20 panels and 4 rows, that 0.9 inch difference in one dimension accumulates to 3.6 inches per row, potentially causing one entire column to not fit on the roof. Always use exact manufacturer dimensions from the panel datasheet, not rounded approximations.
The 3-foot setback default covers most residential US jurisdictions, but exceptions exist. California requires specific 18-inch minimum pathways in some configurations. Hawaii has unique requirements due to high wind exposure. Some HOA-governed communities have aesthetic setback requirements that exceed fire code minimums. Confirm setback requirements with your local AHJ before using this tool's output as the basis for a permit application.
Portrait orientation (panel height vertical) is the default for most residential rooftops and maximizes panel count on wide roofs with limited height. But on narrow, tall roof sections — common on A-frame homes or steep gabled sections — landscape orientation (panel width vertical) may fit more panels. Run this calculator in both orientations for unusual roof shapes to find the configuration that delivers more total panels and kW.
Start by measuring the usable roof section dimensions. Subtract required setbacks (typically 3 feet from all edges) from both width and height. Divide the remaining usable width by (panel width + column spacing) — using the floor() function — to get panels per row. Divide the usable height by (panel height + row spacing) to get number of rows. Multiply panels per row by rows for total panel count. This tool automates all of those steps from your inputs instantly. For complex roof shapes with multiple sections, valleys, dormers, or hips, use a professional tool like Aurora Solar or Helioscope for final permit drawings.
The International Fire Code (IFC) Section 605.11 requires minimum 36-inch (3-foot) access pathways from the roof edge, ridge, and hip ridge for firefighter access. This is the most common requirement across the US for residential solar. California has additional state-specific requirements under Title 24, including 18-inch minimum pathway on the eave side in some configurations. Some HOAs and local municipalities have adopted stricter requirements. Always verify with your AHJ before finalizing a layout — never assume the 3-foot default without confirmation.
A standard 415W residential solar panel in portrait orientation measures approximately 66.9 inches × 40.0 inches (5.57 ft × 3.33 ft = 18.6 sq ft). With 6 inches of row spacing and 2 inches of column spacing, each panel's effective roof footprint is approximately (67+6) × (40+2) inches = 73 × 42 inches = 21.3 sq ft. For a rough estimate, plan for approximately 21–22 square feet of roof area per standard residential panel, including spacing. Higher-wattage panels (430–445W) are slightly larger and may need 22–23 sq ft each.
There is no single "standard" size as module dimensions vary by manufacturer and cell count. However, in 2026 the most common residential module falls in the 410–440W range, measuring approximately 66–68 inches tall by 40–42 inches wide in portrait orientation. Commercial and utility-scale modules have been trending larger, with some 700W+ bifacial panels exceeding 103 inches in height. Always use the specific datasheet dimensions for the module you plan to install, as even a 1-inch difference in panel size affects row count across a multi-row array.
Row spacing affects energy production through inter-row shading — when the shadow of one row of panels falls on the row behind it. On steep-pitch roofs (6:12 or higher), standard 4–6 inch row spacing is sufficient because the panel angle naturally prevents row-to-row shading. On low-pitch roofs (2:12 or less), wider row spacing may be needed to eliminate winter shading losses, but this reduces the number of rows that fit, decreasing total panel count. For flush-mount residential rooftop arrays where all panels are on the same roof plane, row-to-row shading is rarely a significant issue — the concern is primarily relevant for ground-mounted or flat-roof ballasted arrays.
Yes, mixing orientations is technically possible and sometimes necessary around obstructions (skylights, vents, chimneys). However, mixing orientations on the same string can create string voltage mismatches in standard string inverter systems because panel short-circuit and open-circuit voltages may differ slightly by orientation due to temperature differential. Power optimizers or microinverters handle mixed-orientation arrays more gracefully. For string inverter systems, try to keep all panels in a single string in the same orientation when possible.
Professional solar designers use specialized CAD and design platforms for permit-quality layout drawings. The most widely used tools are Aurora Solar (Lidar-based 3D roof modeling, automated layout, shading analysis, string sizing, and proposal generation), Helioscope (strong commercial design capabilities), PVWatts (NREL's free simulation engine), and SolarEdge Designer / Enphase Design Studio for manufacturer-specific system designs. SurgePV's full platform includes AI-powered layout tools that go far beyond this estimator. This calculator is best used for quick feasibility and field estimates before committing time to professional software.
Solar Panel Sizer
Determine recommended system size from energy usage, peak sun hours, and system efficiency before running layout calculations.
Roof Pitch Calculator
Calculate roof pitch from rise and run measurements to determine optimal panel tilt angle and row spacing requirements.
System Size Calculator
Work backwards from your energy usage to find the target system kW — then use the layout estimator to confirm your roof supports it.
Shading Analysis Tool
After planning your layout, estimate annual energy loss from shading and determine whether string inverters or microinverters are appropriate.
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