Clear day solar irradiation question

I have been trying to find out a source of clear sky solar irradiation amounts (direct + diffuse) on a horizontal surface at a particular latitude/longitude on any given date of the year. In particular, I'm trying to figure out the clear sky radiation amounts at Desert Creek, NV (SURFRAD station) with lat/long 36.63,-116.02. If someone can point me in the right direction it'll be greatly appreciated.

A related question: is anyone aware of a simple formula to calculate diffuse radiation on a tilted surface facing south given the diffuse radiation amount on a horizontal surface?

Thanks in advance for any help. Please also copy your response to my email

Best regards,


  • BB.
    BB. Super Moderators, Administrators Posts: 33,523 admin
    Re: Clear day solar irradiation question

    You can use the PV watts program to give you hour by hour data based on ~20 year average historical data.

    The output is based on AC Watts from a grid tied system... But you could work backwards (set derating to 1.00, use watts of panels to assume 1,000 watt/ as STC rating, etc.).

    Or, you can probably download the data sets from the NREL site (and more).

    Here is another thread where somebody wanted to program for an I-Phone/I-Pod... You might try PM'ing him/her

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset