Before any installer visits, you can answer the make-or-break question yourself: is there enough unshaded roof for the system you want? For a 6 kW system — a common size for a typical US home — the arithmetic is simple enough to do from your driveway with a tape measure and a free satellite tool. Here’s the process I’d use, informed by having gone through it at larger scale on my own roof, which carries 40 panels — an exercise that taught me exactly how much “big roof” shrinks once setbacks, vents, and shade get subtracted.
Step 1: How Much Area Does 6 kW Actually Need?
A residential solar panel today is a fairly standardized object: roughly 3.5 feet by 5.5 to 6 feet, or about 18–21 square feet, producing around 400 watts give or take. That makes the core math easy. The table below shows the range — treat the areas as planning examples and check the datasheet of whatever panel you’re actually quoted.
| Panel wattage | Panels for 6 kW | Approx. area per panel | Total panel area |
|---|---|---|---|
| 370 W | 17 | ~19 sq ft | ~325 sq ft |
| 400 W | 15 | ~20 sq ft | ~300 sq ft |
| 430 W | 14 | ~21 sq ft | ~295 sq ft |
Notice the punchline: across the realistic panel range, a 6 kW array lands right around 300 square feet of panels. Higher-wattage panels trim a panel or two off the count but barely change the footprint, because bigger wattage mostly comes from bigger glass.
But panels aren’t the whole requirement. US fire codes generally require access pathways and ridge setbacks — commonly three-foot clear paths, though local rules vary — and every roof has vents, chimneys, and skylights that break up the layout. A practical rule of thumb: look for 350–450 square feet of contiguous, well-oriented, unshaded roof to comfortably fit 6 kW.
Step 2: Measure Your Roof Without Climbing It
You don’t need to get on the roof. Two ground-level methods get you close:
The satellite method. Google Maps’ measurement tool (right-click → “Measure distance”) lets you trace your roof planes from the aerial view and read off square footage directly. Google’s Project Sunroof goes a step further in covered areas, estimating usable solar area and even shade. These measure the horizontal footprint, so a pitched roof has a bit more true surface area than the tool reports — which means the satellite number is conservative, a fine property for a first pass.
The tape-measure method. Measure the house’s footprint under the roof plane you care about, then multiply by a pitch factor (about 1.03 for a shallow 4/12 pitch up to about 1.2 for a steep 9/12). For a first screen, even skipping the pitch correction works — again, it just makes your estimate conservative.
While you’re out there, note the orientation of each candidate plane. In the northern hemisphere, south-facing is best; east and west faces typically produce on the order of 15–20% less annually — usable, but it may mean you need an extra panel or two to hit the same output. North-facing planes are generally not worth using at typical roof pitches.
Step 3: Take Shade Seriously — It Moves
Shade is where driveway intuition fails, because you’re seeing one moment of one day. The oak that’s harmlessly behind the house at noon in July throws a long shadow across the whole south face at 3 p.m. in December, when the sun sits low. Chimneys, plumbing vents, and the neighbor’s second story all do the same on smaller scales.
A useful mental model: for each candidate roof area, ask what blocks the sky along the sun’s winter path, not just overhead. Then check systematically:
- Free: Project Sunroof’s shade analysis where available; sun-path phone apps that overlay the sun’s track on your camera view, letting you stand at roof-corner positions (or sight along them) and see what intercepts the sun in each month.
- Professional: installers measure sun access with dedicated shade-analysis tools that integrate the whole year. As a rough benchmark, they generally want to see something like 90%+ annual sun access on the panel area; persistent shading well below that is a signal to relocate panels or shrink the system.
If only part of the roof has shade issues, panel-level electronics (microinverters or DC optimizers) contain the damage — a shaded panel underperforms alone instead of dragging its whole string down. That’s a mitigation, though, not a cure: electronics can’t create sunlight that a tree is absorbing.
Step 4: Validate With PVWatts Before Anyone Quotes You
Area tells you the system fits. NREL’s PVWatts calculator (free, pvwatts.nrel.gov) tells you what it will produce. Enter your address, 6 kW, your roof’s tilt and azimuth, and it returns monthly and annual production estimates from decades of local weather data.
Two ways to use it:
- Sanity-check the size. Compare PVWatts’ annual output against your usage from twelve months of electric bills. If a 6 kW system is projected to produce far more than you use, and your utility doesn’t credit exports well, a smaller system may pencil out better — and fit more easily on imperfect roofs.
- Stress-test the shade. PVWatts assumes an unshaded array by default. Re-run it with a higher loss percentage representing your estimated shading and see whether the economics still work. If a realistic shade loss turns the project marginal, you want to know that now, not after signing.
When installer quotes arrive, compare their production estimates against your PVWatts run. Reputable installers model shade with real site data and will happily explain differences; a quote wildly above your PVWatts number with no explanation is a red flag.
What Disqualifies a Roof — and What Doesn’t
Usually fine: east/west orientation, a vent or two in the layout, splitting the array across two roof planes, modest morning or evening shade at the array’s edge.
Genuine problems: heavy year-round shade from trees you can’t or won’t trim, a roof needing replacement within a few years (re-roof first — removing and reinstalling panels later costs real money), or usable area well under ~300 square feet, which means accepting a smaller system rather than forcing 6 kW.
The Bottom Line
The screen takes an afternoon: confirm roughly 350–450 square feet of usable, well-oriented roof; walk the winter sun path for shade; run PVWatts to convert area into expected production. Do those three things and you’ll approach installer conversations knowing whether 6 kW genuinely fits your roof — and you’ll recognize both an honest site assessment and an optimistic one when you see them.