Factors Impacting Solar Performance

Factors impacting the performance of a residential and commercial solar system fall into three groups; environmental, site location and equipment.


  • Solar Insolation: average amount of sun hours per day/ month /year at the home/site (on the solar array) changes depending on regional weather, time on year and proximity to the equator.  Peak sunlight hours can range from 3-6 hours per day resulting in significant variance in performance from solar modules.
  • Temperature Factor: power output from the solar module decreases slightly as the temperature rises (due to the properties of the silicon in the module).  Cool, sunny weather is optimal; this includes sites built at altitude.
  • Soiling Factor: when placed outdoors, dust and airborne particles settle on the solar modules; these particles block the amount of light reaching the module cells.  Washing the modules on a regular basis helps solve this issue; frequent rainfall often serves the same purpose.     


  • Orientation of the house/site: modules produce more watt-hours of power when the light source is located perpendicular to the surface of the module.  South facing modules placed at ~15%-30% tilted angle will increase amount of direct (perpendicular) sunlight and therefore improve the performance of the solar modules.
  • Shading around the site: trees, chimneys, roof gables and other buildings that cast a shadow on the solar panels during peak sun light hours will negatively impact performance.


  • Solar Module Rating/Performance: Once operating outside “pure” factory conditions, panels are typically expected to perform at ~85% of the official rating.
  • System Wiring: produces resistance in electrical flow generates losses of ~5%, reducing performance to ~95%.
  • Inverter Efficiency: conversion from DC to AC power results in an energy decrease (in form of heat); inverters are typically rated to work at ~90%. 
  • Total impact of equipment performance factors adds up to ~72% (85%*95%*90%).  This implies that installers have to “overbuild” system by 1.4x to achieve a specific output level.