Based on NREL PVWatts methodology. Default composite: ~87% system efficiency (13% total losses). Adjust shading for heavily shaded sites.
- Select a state to auto-fill peak sun hours
- Default: 400W panels, 21% efficiency
- System efficiency: 87% (NREL PVWatts methodology)
- Roof area includes 25% for panel gaps & spacing
Estimated Monthly Production
Based on typical US irradiance distribution for the selected state
Grid-Tied System
We use the NREL PVWatts DC sizing methodology. Your annual energy target is divided by the expected production per kWp to find the required system size, then divided by panel wattage for panel count.
System Size (kW DC) = Annual kWh Target ÷ (Peak Sun Hours × 365 × System Efficiency)
Number of Panels = CEILING(System Size × 1000 ÷ Panel Wattage)
Off-Grid System
Off-grid arrays must cover daily loads accounting for all conversion losses (inverter, battery round-trip, charge controller, DC wiring). Battery bank is sized for your chosen autonomy days.
Array Size (W) = Daily Load ÷ (Peak Sun Hours × System Efficiency × 0.95)
Battery Bank (kWh) = Daily Load (Wh) × Autonomy Days ÷ (DoD × 1000)
Commercial System
Same methodology as grid-tied but sized from annual consumption, typically with higher derate factors and ILR ratios. Cost estimate uses ±15% range at your specified $/W, with 30% ITC applied.
System Efficiency / Derate
The composite system efficiency multiplies five loss factors: inverter efficiency, DC wiring losses, soiling, shading, and mismatch/other. Default composite ~87% aligns with real-world residential systems. NREL PVWatts default is 86%.
Derate = Inverter% × Wiring% × Soiling% × Shading% × Mismatch%