Solar irradiance statistics (sunshine hitting the ground!)

lasitterlasitter Solar Expert Posts: 56 ✭✭
So there's lots of information about the amount of sunlight reaching the upper atmosphere, but I'm drawing a blank on statistics relative to the amount of sunlight reaching the ground (through cloud cover) at a given weather reporting station for a given day. I'd like to find this information for anywhere near me so that I can compare year over year available sunlight for a given month so I can know if my output has declined because of aging or growth of additional vegetation. Any help in finding a resource for this info would be great.


  • BB.BB. Super Moderators, Administrators Posts: 32,204 admin
    Weather Underground has lots of information and they even support personal weather stations (which can have solar energy sensors):

    Although, I could not find any historical solar irradiance data even though some weather stations do support.

    And there is the old US Government data sets: 

    And this site may give you exactly what you are looking for:

    In general, if you are debugging your system, you could look at the Imp-actual of the array--From what I have seen, Imp-array should range from 50% to 100% of rated value (assuming this is a GT inverter installation where 100% of "available" solar energy is transferred to the AC mains. If you see less than 50% of rated Imp-array on a good day around solar noon--I would suspect that you have an array/wiring/GT inverter issue.

    If you have two or more parallel strings--Measure the current for each string. In general, the strings should match within 10% or better Iarray...

    If you have an off grid / battery based system--That complicates the measurements/estimates a lot... The battery bank your daily loads are a huge influence in actual current/voltage of the array.

    Here is a nice little table that shows High/Low/Average solar insolation for various cities--Shows what the "range" of yearly harvest variability can be over the long term:

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lasitterlasitter Solar Expert Posts: 56 ✭✭
    BB: Thanks. What I'm really looking for is something like, July 2018 vs July 2019, amount of sunlight reaching the ground, so I could adjust for months that were more or less sunny / cloudy.
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