SolarIntermediate
Size a Residential PV Array for Your Roof
Time
60–120 min
Steps
7
Pre-check
4 items
Skill
Intermediate
Scope
Estimate how large a rooftop solar array your home needs and whether your roof can physically hold it. Covers reading your annual kWh usage, finding peak sun hours, measuring usable roof area with code setbacks, accounting for tilt/azimuth and shading, and converting all of that into a panel count and an annual production estimate. This is a desk-and-measurement exercise — no electrical work.
Safety
Read before starting
This is a planning exercise, not an installation. The only physical hazard is measuring on the roof — if you go up, use a stable ladder, fall protection, and never walk a wet or steep roof. You can do the entire estimate from satellite imagery and your utility bills without leaving the ground.
Pre-Check
4 items · complete before you start0 / 32 complete
Steps
Pull your annual electricity usage
- Add up kWh from the last 12 months of bills to get your annual consumption (a typical US home is 9,000–11,000 kWh/yr).
- Note your highest and lowest months — that spread tells you how seasonal your load is.
- If you plan to add an EV, heat pump, or pool soon, add its estimated annual kWh now.
- Divide annual kWh by 365 to get your average daily kWh — you will use this against peak sun hours.
Tips
- Most utility online portals let you download 12 months of usage as a CSV in one click.
Find your location’s peak sun hours
- Peak sun hours (PSH) is the number of hours per day the sun delivers 1,000 W/m² equivalent — it bundles latitude, climate, and average cloud cover into one number.
- Most of the US lands between 4 and 6 PSH; the desert Southwest is higher, the Pacific Northwest and Northeast lower.
- Look up your figure with NREL PVWatts (link in resources) — enter your address and it returns location-specific solar resource.
- Use the annual average PSH for sizing; you can refine by season later.
Code notes
- Irradiance, peak sun hours, and Standard Test Conditions (STC, 1,000 W/m² at 25 °C) are the reference conditions every module nameplate is rated against — real-world output is always a fraction of nameplate.
Measure usable roof area
- Measure each roof face you might use (length × width). Convert to square feet.
- Subtract code-required setbacks: most jurisdictions follow the IRC/IFC fire-access pathways — typically a 3 ft clear path along ridges and sometimes the eaves.
- Subtract obstructions: plumbing vents, chimneys, skylights, and any area within the shadow line of a dormer.
- A standard 60-cell residential module is roughly 18 sq ft (about 41 in × 68 in). Divide net usable area by ~18 to get a rough maximum panel count per face.
⚠ Warnings
- Do not skip the fire setbacks in your estimate — your inspector will, and an array that ignores them gets redesigned after you have already bought panels.
Code notes
- Rooftop fire-access pathways and setbacks come from the local fire code (IFC §1204 / IRC) and the AHJ, not the NEC. Confirm your jurisdiction’s exact pathway widths before finalizing layout.
Adjust for tilt and azimuth
- A south-facing roof (azimuth ~180°) at a tilt near your latitude is the production benchmark — call it 100%.
- East- or west-facing faces typically produce ~80–85% of an equivalent south face; north faces are usually not worth it in the northern hemisphere.
- Low or flat roofs can use tilt-up racking to reach optimal pitch, but tilted rows shade each other — add row spacing, which reduces panel count.
- PVWatts lets you enter tilt and azimuth per face so you can compare candidate roof planes directly.
Account for shading
- Walk the roof’s sightlines at mid-morning and mid-afternoon, or use a shading tool, to find what blocks the sun and when.
- Even partial shade on one panel can drag down a whole series string with a basic string inverter — note this, it drives the inverter decision.
- Trees grow: estimate 10–15 years out, not just today’s canopy.
- Quantify shading as a rough loss percentage (e.g., “west face loses ~15% from the neighbor’s oak after 2 pm”).
Tips
- Heavy or complex shading is the main reason installers choose microinverters or DC optimizers — see the related guide on string vs. microinverters.
Choose module wattage and count the panels that fit
- Pick a representative module wattage — modern residential panels run roughly 380–450 W each.
- Higher-wattage, higher-efficiency panels produce more in the same footprint, which matters on a small or shaded roof.
- Multiply the panel count that physically fits (from Step 3) by the module wattage to get your maximum DC array size in watts.
- Example: 20 panels × 400 W = 8,000 W = 8 kW DC.
Code notes
- Module efficiency is the share of incident sunlight a panel converts to electricity (typically 19–23% today). It sets how much power you get per square foot — the lever that matters most when roof area is the constraint.
Estimate annual production and reconcile with your usage
- Real systems lose energy to heat, wiring, inverter conversion, soiling, and mismatch — apply a system derate of roughly 0.75–0.80 to nameplate.
- Quick estimate: array kW × annual PSH × 365 × derate = estimated kWh/yr. Example: 8 kW × 4.5 PSH × 365 × 0.78 ≈ 10,250 kWh/yr.
- Compare that to your annual usage from Step 1 to see your offset percentage.
- Run the same numbers through the in-app Solar PV Sizing calculator and PVWatts to sanity-check your hand estimate before you commit.
Tips
- Sizing to ~100% of annual usage is common, but check your utility’s net-metering rules first — some pay very little for over-production, which changes the optimal size.
✅Continue Gate:Do you have a defensible panel count, array size in kW, and an annual production estimate that you have cross-checked against both your real usage and PVWatts?