Solar panel layout calculator

How many panels fit on a roof slope, a wall or a carport, what that adds up to in kilowatts and kilograms. Change the numbers and the layout redraws at once.

Area

The space available for panels. Measure a pitched roof along the slope, not across its footprint.

Width, m Slope length, m

Typical areas

Panel Manufacturer Model

Take the dimensions from the datasheet: revisions of the same model differ.

Width, mm Height, mm Power, W Weight, kg

Panel electrical data

From the datasheet — needed for the string calculation. Already filled in for the selected model.

Voc, open circuit, V Vmp, operating voltage, V Isc, short circuit, A Imp, operating current, A Temp. coefficient of Voc, %/°C Temp. coefficient of Pmax, %/°C

Mounting

Gap between panels, mm Edge margin, mm

15–25 mm is a typical mid-clamp width; take the exact figure from your mounting instructions.

The roof edge takes the strongest wind uplift, which is why panels are usually kept away from it.

Different margins per side

Top, mm Right, mm Bottom, mm Left, mm

Try rotating the panel 90° Fill the leftover strip with the other orientation

Obstacles

Chimney, vent, roof window. Distances are measured from the edges of the area: from the left and from the bottom (the eaves).

No obstacles: panels cover the whole area.

Clearance around an obstacle, mm

Flashing, a service path, snow — no panel goes there.

Strings and inverter

How many panels to wire in series for a given inverter, and how many such strings it takes.

Manufacturer Inverter

Design minimum, °C Panel temperature in summer, °C

×1.25 reserve on cold Voc

By the datasheet, at −20 °C the open-circuit voltage rises by roughly 10–12 %, depending on the panel's Voc coefficient. The ×1.25 reserve is the old rule-of-thumb from when that coefficient was −0.35 %/°C and −40 °C was the design case. It always overshoots, but at −20 °C the voltage will certainly not be higher. Overshooting is the safe side when picking an inverter: exceeding the DC maximum destroys it, and that is not a warranty case.

Inverter parameters

Max DC voltage, V Start-up voltage, V MPPT window, from, V MPPT window, to, V Tracker operating current, A Tracker short-circuit current, A MPP trackers Strings per tracker Max array power, kW

Layout diagram

Result

— Panels

— Power, kW

Grid: —
Orientation: —
Occupies: —
Free strip: —
Coverage: —
Removed by obstacles: —
In the shaded zone: —
Panel weight: —

Strings and inverter

— Panels per string

— Strings

Allowed: —
String Voc when cold: —
String Vmp when hot: —
Tracker current: —
Panels per inverter: —
Array power: —
DC / AC: —

This is a preliminary estimate, not a design: the final string layout, protection and cable sizing have to be checked by an engineer.

How the calculator counts

Margins come off the area first: everything else is counted inside what is left.
Panels sit side by side with the gap you set. A row of N panels has N−1 gaps — none are needed at the ends.
With rotation allowed, both orientations are counted separately and the one that fits more panels wins.
Infill fills the strip left over at the side or the bottom with panels in the other orientation — sometimes that is one extra row.
An obstacle's shadow follows the sun: the city latitude, the date and 9:00, 12:00 and 15:00 solar time, projected onto the plane of the slope. A shaded panel stays in the count but is marked — the call is yours.
Strings: the panel's cold Voc times the number in series is checked against the inverter's DC maximum, its hot Vmp against the MPPT window, and the string current against the tracker limit. Of all the string lengths that pass, the calculator picks the one that yields the most power.
The area is a rectangle. For a hipped roof, split it into several rectangles for now and count each one separately.

What the calculator does not cover

Snow and wind loads, and what the rafters can carry: panels are heavy enough that the roof has to be checked.
Shading from trees and neighbouring buildings: the calculator only sees what stands on the area itself.
Protection, cable sizing and fuse ratings: the strings are worked out here, the electrical design is not.
Fire-service access paths and local rules on setbacks from the edge and the ridge.
Azimuth, tilt and weather: this is geometry, not a kWh yield estimate.

This is a preliminary estimate. It answers “how many fit” but does not replace a design: roof loading, mounting, cable sizing and the wiring diagram have to be checked by an engineer.