Solar System Losses Calculator

Calculate total system derate factor by accounting for all loss sources.

Total array STC rating

Module/Array Losses

Typical: 5-15%
Typical: 2-5%
Typical: 0-10%
Typical: 1-3%
Age-related (0.5%/yr typical)
First year: 1-3%

Wiring & Connection Losses

Typical: 1-3%
Typical: 0.5-2%
Connectors, combiner boxes: 0.5-1%

Inverter & System Losses

Typical: 95-98%
Typical: 98-99.5%
If applicable: 1-2%
Downtime: 0.5-3%

Industry Standard Derate Factors

System Type Typical Derate Range
Residential Rooftop0.80-0.850.75-0.88
Commercial Rooftop0.82-0.860.78-0.90
Ground Mount Utility0.83-0.870.80-0.90
Tracking System0.84-0.880.82-0.92

PVWatts Default Values

NREL PVWatts uses these default losses:

  • Soiling: 2%
  • Shading: 3%
  • Snow: 0%
  • Mismatch: 2%
  • Wiring: 2%
  • Connections: 0.5%
  • Light-induced degradation: 1.5%
  • Nameplate rating: 1%
  • Age: 0%
  • Availability: 3%

Total: ~14% losses (0.86 derate)

Optimizing System Performance

  • Regular panel cleaning reduces soiling losses
  • Proper string sizing minimizes mismatch
  • Adequate wire sizing reduces voltage drop
  • Module-level optimization (MLPE) reduces shading impact
  • Quality inverters have 97-99% peak efficiency