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Solar Questions


Why are solar data missing from the daily products?

Solar data are typically missing because the satellite observational data are missing and irretrievable.

What are the maximum and minimum temperatures for the hourly temporal level?

The hourly temporal level does not have maximum and minimum temperatures, as the lowest temporal level for the assimilation model is itself hourly.

Tilted Surface

Are the solar minimum and maximum insolation values on a tilted surface available from the POWER data archive?

Yes. The climatologically monthly averaged maximum and minimum solar irradiances incident on equatorial facing tilted surfaces are available through the POWER Data Services for a surface tilted at an angle equal to the user requested latitude ±15°. See the Tilted Solar Irradiances Methodology for a more detailed discussion of how the POWER data compares to the old values from the old SSE web site.

Is the data in Solar Irradiance for Equator Facing tilted surfaces suitable for a tilted panel (or surface) fixed during the day, or is it more suitable for tilted surface with vertical axis rotation (From east to west)?

The POWER solar data incident on tilted panels is only valid for a fixed tilted surface. The SI_EF_Optimum solar insolation is only valid for fixed surfaces at the optimum tilt angle. Currently the POWER site does not provide the solar insolation for surfaces oriented East or West of the vertical axis.

Is the DNI (i.e. DNR) the same as the Optimal Solar Irradiance (i.e. SI_EF_OPTIMAL) for an Equator Facing Tilted Surface?

DNR is the solar insolation incident on a surface at a normal angle (e.g. 90°). SI_EF_OPTIMAL, as well as the remainder of the solar tilt insolation values, are characterized as "All-Sky"-Insolation". The All-Sky-Insolation = (DNR) x COS(THMT) + DIFF where THMT is the solar zenith angle and DIFF is the diffuse insolation resulting from atmospheric scattering of the solar insolation as it passes through the earth’s atmosphere. See the Solar Insolation of Methodology for a more detailed explanation of the POWER method.

Solar Insolation

Do the solar insolation values include cloudiness?

The all-sky values do include cloudiness. The clear-sky values are determined for various cloud 'free' conditions.

Are sunshine data (sunshine hours /day; or percentage insolation) and Solar Radiation Global available in POWER?

Yes. The annual averaged global solar radiation and the monthly averaged daylight hours are available via the POWER Data Services in the Sustainable Buildings (SB) community.

Does the POWER solar insolation represent net radiation or solar radiation requiring an additional cloud term?

The POWER solar values are the downward solar radiation reaching the earth's surface for all-sky conditions. The influence of clouds has been taken into account in computing the solar values. The POWER data does not provide the net radiation defined as difference between the incoming and outgoing radiation.

Is it possible to build a solar insolation schedule per the hour of the particular day?

The POWER archive does not provide hourly solar insolation, however the following comments are offered relative to the application of hourly solar data. The true hourly series is most useful for system management and analysis to assess the output of the solar system relative to the time of day. This is particularly useful if the building is connected to the grid and the utility has different charges by hour. The HOMER decision support tool does require hourly data for a "year" but it focuses on integrating solar, wind, and other conventional power source (i.e. diesel, propane, etc.) for off grid and on-grid systems. HOMER downloaded the old SSE monthly averaged data set and used statistical models to create hourly data for a "year". HOMER also uses hourly typical design years. Currently, the geosynchronous data that POWER solar irradiance is derived from does not have hourly data before 2000 and thus we do not produce hourly data for the long-term series. POWER may consider adapting an hourly data set from another data source but this has not been completed. We should note that the RETScreen Expert design package does use daily data and long-term monthly data for it’s data management and feasibility design packages. Thus, the current POWER data holdings are readily sufficient to support RETScreen functionality, if using RETScreen is an option for the user. Lastly, we do provide climatological averages of the 3-hourly solar irradiance by month and location. With this information a user can better understand, at least to a 3-hourly interval how the solar diurnal cycle varies on average in that location.

Is 'All Sky Insolation Incident on a Horizontal Surface' from Version 1 the same as 'All Sky Surface Shortwave Downward Irradiance' from Version 2?

Yes, the All Sky Insolation Incident on a Horizontal Surface from Version 1 is same as "All Sky Surface Shortwave Downward Irradiance" from Version 2. We updated the names of the parameters to better align with standards, community best practices, and greater consistency across our application. The data values have changed since we are now using SRB v4 and CERES Syn1deg as radiation data sources. See:

Why are the 'Daily' values for a point location different between two sources (SRB/FLASHFlux and MERRA-2)?

For all temporal levels the single point tool pulls the data for the correct location relative to the source data model grid. In this example, SRB/FLASHFlux have a resolution of ½x½ while MERRA-2 have a revolution of 0.5 x 0.625. This allows us to avoid altering the data by re-gridding; keeping it the same as the source allows for better tractability.