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Roof-space estimator
Updated August 6, 2026 · using standard 400 W module-footprint figures · by Cali Energy
How much roof space do you need for solar? Plan on about 50 sq ft of usable roof per kW — roughly 20 sq ft per 400 W panel. So a 6 kW system (about 15 panels) needs ~300 sq ft of usable area. Usable roof is well below your total roof once you remove fire-code setbacks, vents, skylights and shaded zones.
✓ Uses standard module footprints (~20 sq ft per panel, ~50 sq ft per kW). See every figure it uses ↓
~50 sq ft of usable roof per kW; ~20 sq ft per 400 W panel.
Usable area excludes fire-code pathways/setbacks, vents, skylights and shaded zones. A site survey is the only way to know for sure.
The rates and figures this calculator usestap to verify
The tool converts roof area into a system size using standard 400 W module footprints, with an optional factor to turn a total-roof number into usable roof. Here is every figure it uses.
| Usable roof per kW | ~50 sq ft |
| Area per 400 W panel | ~20 sq ft |
| Total → usable factor | ×0.60 — Cali Energy planning assumption (when “total roof” is ticked) |
| Usable area | your area, ×0.60 if it’s total roof |
| Panels | usable ÷ 20, rounded down |
| System size (kW DC) | panels × 0.4 — so the kW always matches the panel count |
Footprints are typical for 400 W modules — high-efficiency panels pack a little tighter, budget panels a little looser. Usable roof excludes fire-code pathways and setbacks, vents, skylights, chimneys and shaded areas; the ×0.60 total-to-usable factor is a rough planning haircut, not a code calculation. A site survey is the only way to confirm real capacity.
Example results you can reproduce
Each row enters a usable roof area (the “total roof” box left off) — enter the same value above to reproduce it.
| Usable roof | System size | Panels (400 W) |
|---|---|---|
| 300 sq ft | 6.0 kW | 15 |
| 500 sq ft | 10.0 kW | 25 |
| 700 sq ft | 14.0 kW | 35 |
Tick “this is my total roof” and a 1,000 sq ft total becomes ~600 sq ft usable — about a 12 kW system, or 30 panels.
How this calculator works
It starts from the roof area you enter. If you tell it that’s your total roof, it applies a rough ×0.60 factor to approximate the usable portion. It then divides usable area by ~20 sq ft to get the number of 400 W panels that fit, and multiplies that count by 0.4 kW for the installed system size — so the kW and the panel count always agree (about ~50 sq ft per kW). These footprints assume standard 400 W modules laid out with normal spacing. It’s a planning estimate, not a stamped layout.
How to read your result
You get an approximate system size in kW and the number of 400 W panels that fit. Treat it as a ceiling on what the area could hold, not a guarantee — real layouts lose space to obstructions, orientation and the need to group panels on the sunniest planes. If you entered total roof, the usable figure it worked from is shown too.
What moves the result the most
Usable area is everything. The single biggest error is entering total roof as if it were usable — a complex, cut-up or partly shaded roof can leave far less than half its area truly usable, while a big, simple, south-facing plane loses little. Panel efficiency matters at the margin: higher-wattage modules fit more kW into the same square footage.
Common questions
What counts as “usable” roof?
Clear, reasonably sun-facing roof area left after removing fire-code pathways and setbacks, vents, skylights, chimneys, dormers and heavily shaded sections. It’s often well under half of the total roof.
Why is usable roof so much less than total?
Codes require access pathways and edge setbacks for firefighters, obstructions break up the plane, and panels only belong on the sunnier faces. All of that is space you can’t (or shouldn’t) fill with modules.
How much roof do I need per kW?
About 50 sq ft of usable roof per kW with standard 400 W panels — so a 6 kW system needs roughly 300 sq ft, and a 10 kW system about 500 sq ft.
Can I fit the system I want on my roof?
Enter your usable area and compare the kW it supports to the size you’re targeting. If it comes up short, higher-wattage panels or using more roof faces can help — or a battery lets a smaller array still cover evening use.
Do skylights, vents and AC units count against me?
Yes — each one blocks panels and forces spacing around it, so a roof dotted with obstructions yields less usable area than its square footage suggests. That’s what the total-to-usable factor approximates.
Is this a real roof layout?
No — it’s an area-based estimate. A designer lays out actual panels on your specific roof planes with shading analysis and code setbacks, which is the only way to know the true count.
Sources
- NREL — PVWatts Calculator
- 2025 California Fire Code, Chapter 12 §1205 (solar access & pathways)
- LAFD — Solar Power Placement Requirements (local)
Estimates only, not a quote. Assumptions, rates, incentives and program rules can change — confirm current details with your utility, the program administrator or a licensed professional.
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