Solar & battery
Solar Roof Area Calculator
Works the usable roof area, which is well below the measured roof.
Result
Usable roof is always smaller than measured roof. Fire code setbacks along ridges and eaves, access pathways, vents, chimneys, skylights and anything that casts a shadow all take space, and a quarter to a third is a normal loss.
Get a Free Business AuditHow the calculation works
Each roof plane's area reduced for fire access setbacks, obstructions such as vents, chimneys, skylights and plumbing stacks, and the module layout's own spacing, giving the area actually available and the array size it supports. The usable figure is the one that determines what can be installed.
What moves the number
- Required fire access pathways and setbacks.
- Obstructions on each plane.
- Plane orientation, since some contribute little.
- Module dimensions and the layout they allow.
Where this one goes wrong
Fire access requirements are set by the adopted building and fire code and enforced by the local authority, and they differ between editions and jurisdictions, so the setbacks have to come from the local requirement rather than a remembered rule. They remove a substantial share of a typical roof. Obstructions also remove more than their own footprint, because a layout has to work around them in whole modules rather than in square feet.
Before you quote
- Get setback requirements from the local adopted code.
- Lay out in whole modules, not in area.
- Exclude planes whose orientation contributes little.
Where the rules come from
Questions
About this calculator
Why is usable roof area so much less than the roof?
Fire access setbacks, vents, chimneys, skylights and stacks all remove area, and planes facing away from the sun contribute little. Obstructions also remove more than their footprint, since the layout works in whole modules rather than square feet.
What setbacks are required?
Whatever the locally adopted building and fire code requires, which differs between editions and jurisdictions and is enforced by the local authority. A remembered rule is not a substitute for the requirement actually in force where the work is.
Can every roof plane be used?
Physically often yes, usefully not always. A plane facing away from the sun produces so little that the installed cost rarely justifies it, which is a design decision rather than a limitation, and it is worth making deliberately.
The formula
area = panels x area per panel; panels = system watts / panel watts
Put this calculator on your site
Free to embed. Paste this where you want it to appear. It stays up to date automatically because it loads from BDEVY.
Please keep the attribution line. It is the only thing we ask in return.
How to use this calculator
- Work out. Enter your own figure; the page starts at Area needed for a system size.
- System size (kW DC). Enter your own figure; the page starts at 7.2.
- Usable roof area (sq ft). Enter your own figure; the page starts at 700.
- Panel rating (watts). Enter your own figure; the page starts at 400.
- Area per panel (sq ft). Enter your own figure; the page starts at 21.
- Roof lost to setbacks and obstructions (%). Enter your own figure; the page starts at 25%.
The result updates as you type, so you can try a figure, see what it does to result, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Work out | Area needed for a system size |
| System size (kW DC) | 7.2 |
| Usable roof area (sq ft) | 700 |
| Panel rating (watts) | 400 |
| Area per panel (sq ft) | 21 |
| Roof lost to setbacks and obstructions | 25% |
What comes out
| Result | Value |
|---|---|
| Result main result | 378.0 |
| Panels | 18 |
| Area needed | 378 |
| Capacity that fits | 10.0 |
| Usable roof after setbacks | 525 |
A 7.2 kW system is 18 panels needing 378 square feet of unshaded, unobstructed roof.
How sensitive the answer is
Holding everything else steady and changing only system size (kw dc), this is what happens to result. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| System size (kW DC) | Result |
|---|---|
| 3.6 | 189.0 |
| 5.4 | 294.0 |
| 7.2 example above | 378.0 |
| 10.8 | 567.0 |
| 14.4 | 756.0 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Work out
- Choose from Area needed for a system size, Capacity a roof will take.
- System size (kW DC)
- A number, between 0 and 5000, starting at 7.2.
- Usable roof area (sq ft)
- A number, between 0 and 500000, starting at 700.
- Panel rating (watts)
- A number, between 0 and 1000, starting at 400.
- Area per panel (sq ft)
- A number, between 0 and 60, starting at 21.
- Roof lost to setbacks and obstructions (%)
- A percentage, between 0 and 80, starting at 25%.
What the results tell you
| Result | What it is |
|---|---|
| Result | The headline result. A decimal figure. |
| Panels | A number. |
| Area needed | A number. |
| Capacity that fits | A decimal figure. |
| Usable roof after setbacks | A number. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
Keep going
Related calculators
Estimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.