The system-sizing formula
Recommended system size (kW) = (annual kWh usage × target offset %) / annual kWh per kW production
- Annual kWh usage: 12 months of utility bills, typically 8,000-18,000 kWh for residential.
- Target offset: 80-110% of usage. NEM rules influence the right target.
- Production per kW: varies by sun region (1,100-1,800 kWh/kW/year depending on latitude + climate).
Example: home using 14,000 kWh/year × 100% target / 1,400 kWh/kW/year = 10 kW recommended system.
Constraints that modify the math
- Usable roof area. A 10 kW system requires roughly 500-700 sq ft of usable south-facing roof at standard panel efficiency. Smaller homes may be roof-constrained below their usage offset target.
- Shading. Mature canopy or adjacent structures reduce effective production per kW. Heavy shading can drop production by 30-50%.
- Net metering rules. Under reduced-rate NEM (e.g., NEM 3.0), oversizing past usage doesn't pay back. Right-sizing to 80-90% of usage with paired battery is often optimal.
- Electrical panel capacity. Older 100A panels may need upgrade ($1,500-$4,000 addon) to support 10+ kW systems.
How Solar Launch auto-calculates system size
Every Solar Launch render attaches an auto-calculated system size in kW. The math uses:
- Google Solar API rooftop geometry for usable south-facing area.
- Per-facet sun exposure modeling across an annual cycle.
- Standard panel-density assumptions to convert area to kW.
- Local production-per-kW assumptions to model annual kWh.
The auto-estimate is within 5-10% of a hand-engineered design for typical single-family pitched roofs — accurate enough to mail a postcard with a real production + savings number on the customer portal.
System size + production + savings on every render.
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