Solar Roof Area Calculator: How to Size Panels from Satellite Imagery

For a solar installer, the right number isn't the roof area — it's the usable panel area. Subtract obstructions, factor in tilt, and account for shading, and you'll land on a system size that actually works. Here's the calculator method that gets you within ±5% before you ever climb a ladder.

Step 1 — Measure each roof face separately

Panels go on one plane at a time. Use the polygon tool in RoofMate to outline each north-, east-, and west-facing face individually. South-facing faces are usually a no-go in Australia (low irradiance) so skip unless customer specifically requests.

Step 2 — Apply pitch factor for true surface area

Plan area × pitch factor = actual surface area. See our pitch factor table — at 25° pitch, multiply by 1.103.

Step 3 — Subtract obstructions

Use the polygon tool to outline each obstruction, then subtract from the face area:

• Skylights and roof windows
• Whirlybirds and roof vents
• Antennas and satellite dishes
• Chimneys and flues
• Aircon condensers and ducting
• Hot water service
• Plumbing penetrations (more common than you'd think — add a 0.5 m exclusion zone around each)

Step 4 — Apply setback margins

AS/NZS 5033 requires:

• 200 mm setback from gutter edge
• 200 mm setback from ridge line
• 1 m setback from hip lines on each side (some jurisdictions vary)
• 500 mm walkway between arrays for maintenance access

Practically: deduct ~15% of the raw face area for setbacks on a typical hip roof.

Step 5 — Calculate panel count

Standard residential panel today: 1.7 m × 1.1 m = 1.87 m². Tilt frames not included (assume coplanar mount). Example calculation:

• Plan area of north face: 60 m²
• × pitch factor 1.103 (25°): 66.2 m²
• − 6 m² obstructions: 60.2 m²
• × 0.85 (setbacks + walkway): 51.2 m²
• ÷ 1.87 m² per panel: 27 panels
• × 440 W per panel: 11.88 kWp system

Step 6 — Check shading

A 12 kWp system isn't 12 kWp if it's in the shade. Use Street View, satellite imagery taken at different times of day, or tools like SunCalc to model the shadow path of nearby trees and buildings. Areas shaded for >2 hours/day in winter should be excluded from your panel layout.

Common mistakes that cost installers money

• Forgetting the ridge cap setback — costs 1–2 panels
• Ignoring chimney shadow paths — array under-performs in winter
• Counting panels on south-facing faces — system fails owner's expected payback model
• Not allowing for shade from neighbour's tree — they'll grow another metre in 3 years
• Forgetting cable run length to inverter — a 30 m run can mean a thicker (more expensive) DC cable

Why measure before site visit?

A solar installer who can deliver a same-day quote with an accurate panel count converts 30–50% more leads than one who books a site visit first. Satellite measurement is the difference between "we'll be in touch" and "send through the contract".