Solar Panel Calculator: How Many Panels Fit on Your Roof?

Use this free interactive solar panel calculator to find out how many solar panels fit on your roof. Draw your roof outline on the satellite map, choose your panel size, and the tool instantly fills in a grid of panels — then calculates estimated capacity, annual energy output, and CO2 savings. Works for homes and businesses in the UK, Australia, New Zealand, the US, and anywhere in the world.

How to Use the Solar Panel Calculator

Step 1: Find Your Property

Type your address, postcode, or suburb into the search bar and click Search. The map will zoom to your property using satellite imagery so you can see the roof clearly.

Step 2: Draw Your Roof Outline

Click Draw Roof, then click around the edges of your roof to trace its outline. Double-click to finish the polygon. The tool works on any roof shape — rectangular, L-shaped, or irregular. If you have multiple roof sections, draw the one you want to assess first.

Step 3: Configure Your Panels

Choose from three standard panel sizes — Standard UK/AU/NZ (1.72 x 1.04 m), US Standard (1.65 x 0.99 m), or Large Format (2.0 x 1.05 m) — or enter custom dimensions. Toggle between portrait and landscape orientation and set the gap between panels (default 20 mm). Switch between metric and imperial units at any time.

Step 4: Adjust the Layout

The calculator automatically fits as many panels as possible inside your drawn area. Click any individual panel to remove it — useful for marking chimneys, skylights, roof vents, or other obstructions. Removed panels are greyed out and excluded from all calculations.

Step 5: Export and Share

Click Share Link to copy a URL that preserves your exact roof outline, panel configuration, and any removed panels — useful for sharing with a solar installer or a friend. Click Download KML to export the roof polygon and panel layout as a KML file you can open in Google Earth, QGIS, or any GIS application.

Understanding the Estimates

The calculator displays five figures for any completed layout:

• Panels — the number of panels that fit inside your drawn roof area (after any manual removals).
• Roof Area — the footprint of your drawn polygon in m² or ft².
• Estimated Capacity (kWp) — total peak power based on panel count and the wattage setting. A typical modern residential panel is 400–430 W.
• Estimated Annual Output (kWh/yr) — rough annual energy production, calculated as: capacity (kWp) x 3.5 peak sun hours/day x 365 x 0.75 performance ratio.
• CO2 Avoided per year — estimated tonnes of CO2 offset, using 0.20 kg CO2 per kWh as a rough global average.

These figures are indicative only. Actual output depends heavily on your location (peak sun hours range from around 2.5 in northern UK to over 5.5 in parts of Australia and the US Southwest), roof pitch, orientation (south-facing in the northern hemisphere, north-facing in the southern hemisphere), shading from nearby trees or buildings, and the specific inverter and installation used. Always get a quote from a qualified solar installer for site-specific assessments.

Solar Panel Sizes: UK, Australia, New Zealand, and US

Most residential solar panels sold globally fall into a narrow size range. Standard panels used across the UK, Australia, and New Zealand are typically around 1.72 m x 1.04 m (roughly 67 x 41 inches), with 60 or 72 cells. US residential panels are often slightly smaller at around 1.65 m x 0.99 m. Large-format panels — increasingly common in newer installations — measure around 2.0 m x 1.05 m and offer higher output per panel, which can mean fewer panels are needed for the same capacity.

Panel orientation also affects how many fit on a roof. Portrait mounting (tall) is the most common for sloped residential roofs. Landscape mounting (wide) is often used on flat commercial roofs. The calculator lets you toggle between both to see the difference.

What Affects How Many Solar Panels Fit on a Roof?

Several practical factors limit the usable roof area beyond the bare footprint:

• Obstructions — chimneys, skylights, roof vents, dormer windows, and satellite dishes all eat into available space. Use the click-to-remove feature to exclude these areas.
• Setback requirements — many installers leave a gap around the roof edge (often 0.3–0.5 m) for maintenance access, fire safety, and structural reasons. You can account for this by drawing a slightly smaller polygon than the full roof edge.
• Roof pitch — this calculator works on the horizontal footprint drawn on the map. A steeper roof has more actual surface area, but solar installers generally work from the horizontal projection when quoting, so the footprint figure is a practical starting point.
• Structural capacity — older roofs may require structural assessment before a full array is installed. A structural engineer or installer can advise.

Frequently Asked Questions

How accurate is this solar panel calculator?

The panel count is geometrically accurate based on your drawn polygon and the panel dimensions you specify. The energy and CO2 estimates are rough indicative figures — they use a single conservative assumption for peak sun hours (3.5 kWh/m²/day) regardless of location. For a property in Edinburgh this may slightly overestimate output; for a property in Queensland it will significantly underestimate it. Use the figures for a general sense of scale, not for project planning.

Can I use this for a flat commercial roof?

Yes. Draw the roof polygon, switch orientation to Landscape if needed, and adjust the gap between panels — commercial installations often use larger ballasted frames with bigger inter-row spacing (0.5–1.5 m) to avoid shading. Increase the gap setting to reflect this.

What is kWp?

kWp stands for kilowatt-peak — the rated output of a solar array under standard test conditions (1000 W/m² irradiance, 25°C cell temperature). It is the standard unit used to specify and quote solar installations. A typical UK semi-detached home installs 3.5–4 kWp; a larger Australian home might have 6–10 kWp.

Does the calculator account for roof pitch?

No. The calculator works from the horizontal roof footprint visible on the satellite map. Roof pitch is not factored into the panel count or area, since installers typically quote from the plan footprint. A very steep roof will have more actual surface area than shown, but standard residential pitches (15–35 degrees) do not change the panel count significantly.

Can I share my roof layout with a solar installer?

Yes — use the Share Link button to copy a URL that encodes your entire layout. You can also use Download KML to export the roof polygon and panel positions as a KML file, which any installer with Google Earth or GIS software can open.

What units does the calculator support?

Switch between metric (metres, m²) and imperial (feet, ft²) using the Units control. Panel dimensions for the presets are defined in metric; when imperial is selected, roof area is shown in ft². The custom panel size option accepts dimensions in metres (metric mode) or inches (imperial mode).