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How many panels do I need
Updated August 6, 2026 · based on NREL PVWatts California production data · by Cali Energy
How many solar panels does a California home need? A typical home using about 900 kWh a month needs roughly a 6.8 kW system — about 17 panels at 400 W — to cover 100% of its use, assuming ~1,600 kWh per kW-year of production. Your count shifts with usage, roof shading, panel wattage and how much of the bill you want to offset.
✓ Built on NREL PVWatts production data for California — not a number we made up. See every figure it uses and check the source ↓
Production figures are per city from PVGIS for a 20° south-facing roof with 14% system losses. Shading is applied as a production haircut. Module area is computed from real panel dimensions (a REC Alpha Pure-RX 460 W is 1,728 × 1,205 mm, 22.4 sq ft) and scaled by wattage — your roof needs meaningfully more than that once fire setbacks and walkways are included. Planning estimate, not a design.
The rates and figures this calculator usestap to verify
Every figure below is either drawn from an official source or clearly flagged as a Cali Energy planning assumption. The panel count comes from your usage, a California production figure from NREL PVWatts, a panel wattage, a shading factor and a panel footprint — the production table links to its source.
| Energy per kW installed, per year | ~1,600 kWh (typical range 1,500–1,700) |
| Basis | Fixed roof-mount, standard system losses |
| Panel wattage options | 350 / 400 / 440 / 500 W (default 400 W) |
| Roof area per panel | ~20 sq ft, before setbacks |
| None / open roof | ×1.00 |
| Partial (some trees / dormers) | ×0.85 |
| Heavy (significant obstructions) | ×0.70 |
| Assumed all-in rate | $0.35 / kWh |
| Used | Only when you enter a dollar bill instead of kWh |
| Annual target | monthly kWh × 12 × offset % |
| System size (kW DC) | annual ÷ (production × shading) |
| Panels | system kW × 1000 ÷ panel watts, rounded up |
| Roof area | panels × ~20 sq ft |
Production is NREL PVWatts modeling for California — hot inland areas run higher and the coast lower, so the tool shows a 1,500–1,700 kWh/kW range around the estimate. The panel count is a planning figure: a real design depends on your roof planes, azimuth, shading and equipment. The $0.35/kWh bill conversion is a typical California all-in average (total bill ÷ kWh) and varies by utility, plan and tier. The shading factors (×0.85 partial, ×0.70 heavy) and the ~20 sq ft panel footprint are Cali Energy planning assumptions, not code or site-specific values.
Example results you can reproduce
Each row uses 400 W panels, an open (unshaded) roof and a 100% offset target — enter the same values above to reproduce them exactly.
| Monthly usage | Panels | System size | Roof area |
|---|---|---|---|
| 600 kWh | 12 | 4.8 kW | ~240 sq ft |
| 900 kWh | 17 | 6.8 kW | ~340 sq ft |
| 1,200 kWh | 23 | 9.2 kW | ~460 sq ft |
How this calculator works
It turns your monthly usage into an annual kWh target (times your offset slider), then divides by California production — about 1,600 kWh per kW of panels each year, from NREL PVWatts — to get the system size in kilowatts. Dividing that by your panel wattage and rounding up gives the panel count, and multiplying panels by ~20 sq ft estimates roof area. Shading trims production (×0.85 partial, ×0.70 heavy), which raises the count. If you enter a dollar bill instead of kWh, it first converts at ~$0.35/kWh. These are Cali Energy planning calculations, not a stamped design.
How to read your result
You get three numbers: panels, system size in kW DC, and approximate roof area. Beneath them, a range line shows how the count moves between California’s sunnier and cloudier ends (about 1,700 vs 1,500 kWh/kW-year). Treat the panel count as a starting point — the kW size is the more portable number to compare against installer quotes.
What moves the result the most
Four inputs dominate: your usage (or bill), the offset target (covering 80% instead of 100% cuts the system by about a fifth), panel wattage (500 W panels need roughly 20% fewer than 400 W for the same size), and shading — a heavily shaded roof needs about 40% more panels to reach the same output. Local sun matters too, which is why the tool shows a range rather than a single number.
Common questions
What do I need to use this calculator?
Your average monthly usage in kWh (from a recent LADWP or SCE bill), or your monthly bill in dollars. Then pick a panel wattage and set how much of your usage you want the panels to cover.
How many panels does a typical California home need?
For a home using about 900 kWh a month, roughly 16–18 panels at 400 W — about a 6.8 kW system — to cover 100% of usage. Larger or all-electric homes need more.
Should I size for 100% of my usage?
Not always. Under LADWP’s net metering you get near full retail credit for exports, so 100% often makes sense; under SCE’s NEM 3.0 net billing exports are worth less, so many homes pair a smaller system with a battery. Use the offset slider to compare.
Do bigger panels mean fewer panels?
Yes. A 500 W panel produces about 25% more than a 400 W one, so you need fewer of them for the same system size — though the kW and roof area come out similar.
Why does shading add panels?
Shade lowers each panel’s output, so the tool derates production (×0.85 partial, ×0.70 heavy) and adds panels to compensate. In practice, heavy shade is usually better solved by re-siting or trimming than by adding modules.
Is this the number an installer will design?
No — it’s a planning estimate. A real design accounts for your specific roof planes, orientation, tilt, obstructions, code setbacks and the exact equipment, which can move the count up or down.
How much roof space will I need?
Roughly 20 sq ft per panel before setbacks — so a 17-panel system needs about 340 sq ft of usable roof. Usable roof is always less than total roof once you remove edges, vents and shaded areas.
Sources
- PVGIS (European Commission JRC) — PV performance tool, per-city modelling used for the production table
- NREL — PVWatts Calculator (the other standard production model)
- REC Group — Alpha Pure-RX datasheet (460 W module dimensions)
- CPUC — Net Billing Tariff (NEM 3.0)
- IRS — Residential Clean Energy Credit
- U.S. EIA — Electric Power Monthly, average residential price by state
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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