If you want a quick, reliable answer, there are four ways to size a roof for solar: satellite measuring, measuring from architectural plans, measuring in person, and drone survey. Below we go into detail about each method. Read this blog if you want more than a rough guess.
Quick answer
- Typical modern panel footprint: about 1.75 m by 1.1 m in portrait, or 1.1 m by 1.75 m in landscape.
- Leave 0.2 m to 0.3 m setbacks from ridge, eaves and verges, and space around rooflights, chimneys and soil vents.
- On many three-bed semis you’ll fit 8 to 12 panels per unobstructed roof face. That is roughly 3.2 kWp to 5.4 kWp with 400 to 450 W modules.
- Exact count depends on usable area, orientation, obstructions, rafter positions, and your chosen module orientation.
Method 1: Satellite measuring
Best for first-pass feasibility on houses.
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Open a satellite mapping tool with measuring capability.
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Trace the usable roof rectangle on the face you plan to use.
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Subtract setbacks: typically 0.2 m to 0.3 m from ridge, eaves and verges.
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Mark out no-go rectangles around rooflights, chimneys and flues, and keep a practical service gap of at least 0.2 m around them.
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Note the remaining length and height of the clear zones.
Method 2: Measuring from plans
Best for new builds, extensions or re-roofs.
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Use the latest scaled roof plan and section drawings.
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Read span, slope length and verge-to-verge dimensions directly.
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Apply setbacks and exclusion zones as above.
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Decide panel orientation now, because portrait vs landscape changes rail spans and count.
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Check any planning constraints or conservation details that dictate layout symmetry.
Method 3: Measuring in person
This is where we convert desk work into a buildable array.
At Spirit Energy we want to ensure our customers get the most accurate quote. That is why Justin our Head of Installations personally goes to each site in order to complete a technical survey, after which we commission an external chartered structural survey. Between them we confirm:
- Usable dimensions and real-world obstructions.
- Rafter size, spacing and direction for safe rail fixing.
- Roof covering type and condition.
- Loads and fixings to meet uplift and sliding checks.
- Cable routes, isolator locations, earthing, and inverter siting.
Method 4: Drone survey
Useful on complex or tall buildings, and common for our commercial work, for example in Cobham Service Station or Philip Dennis Foodservice.
- High-resolution orthophotos let us measure accurately and place modules around obstructions.
- We capture parapet heights, plant locations, and shading from adjacent structures.
- On flat roofs we also plan inter-row spacing to avoid self-shade on tilted frames.
Watch: Sam’s Solar Journey on panel count
This is part 1 of our Sam's Solar Journey series on our YouTube channel. In this episode, we walk you through the various methods of our design process, offering an insightful behind-the-scenes look at the techniques involved in providing you with the most accurate output information.
Watch this video to find the answer to how many solar panels can fit on your roof:
Use the video playlist to learn everything about the solar journey with Spirit Energy.
Spirit's Solar Calculator
If you’d like to get a quick idea of how many panels will fit on your roof, try our solar calculator. Simply enter your postcode, roof type, and available space, and our tool will estimate how many panels you can install, the system size in kWp, and the potential generation and savings. It’s a fast way to explore options before booking a full survey, and it gives you a realistic picture of what solar could deliver for your home.
What happens next
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Once you request a quote, one of our technical designers will run a satellite assessment on your roof.
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We’ll propose a layout and kWp with expected generation.
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Our Head of Installations will complete an on-site technical survey, and we will arrange an external chartered structural survey.
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We finalise stringing, fixings and cable routes, then install to MCS standards and handle your DNO application.
FAQ
Q: Can I just pack panels edge to edge?
A: No. Leave sensible margins to edges, allow for clamps, and keep access space around obstructions.
Q: Portrait or landscape?
A: Whichever yields the higher count with clean fixing into rafters. Landscape often wins on short, wide roofs. Portrait suits tall, narrow faces.
Q: Do hips, valleys and dormers ruin layouts?
A: They reduce the usable rectangle. A drone or on-site survey lets us tessellate modules neatly around them, often mixing orientations.
Q: What about in-roof systems?
A: In-roof modules replace tiles, so the footprint is similar but detailing, ventilation and structure differ. Count method is the same, but design rules change.
If you'd like us to assist with verifying your calculations, get in touch with our team today.
