Solar Questions¶

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

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

Yes. The climatologically monthly averaged maximum and minimum solar irradiances incident on equator-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 POWER data compares to the old values from the old SSE web site.

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.

DNR is the solar insolation incident on a surface at a normal angle (e.g. 90°). SI_EF_OPTIMAL, along with the other solar tilt insolation values, are characterized as "All-Sky"-Insolation". The All-Sky-Insolation is calculated as: (DNR) x COS(THMT) + DIFF, where THMT is the solar zenith angle, and DIFF is the diffuse insolation resulting from atmospheric scattering of 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.

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

Yes. Annually averaged global solar radiation and monthly averaged daylight hours are available via the POWER data services under the Sustainable Buildings (SB) community.

The POWER solar values are the downward solar radiation reaching the Earth's surface for all-sky conditions. The influence of clouds is accounted for when computing the solar values. POWER data does not provide net radiation, which is defined as difference between the incoming and outgoing radiation.

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 sources (i.e. diesel, propane, etc.) for both 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 from which POWER solar irradiance is derived does not have hourly data prior to 2000. As a result, 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. It is worth noting that the RETScreen Expert design package uses daily and long-term monthly data for its data management and feasibility design packages. Therefore, the current POWER data holdings are sufficient to support RETScreen functionality, if using RETScreen is an option for you. Lastly, POWER provides climatological averages of three-hourly solar irradiance by month and location. This information helps you better understand how the solar diurnal cycle varies on average in the given location, at least to a 3-hourly interval.

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. The names of the parameters were updated to better align with standards, community best practices, and greater consistency across our applications. The data values have changed, as we are now using SRB v4 and CERES Syn1deg as radiation data sources. See: https://power.larc.nasa.gov/docs/methodology/data/#solar.

For all temporal levels, the single point tool pulls data for the correct location relative to the source data grid. In this example, SRB/FLASHFlux has a resolution of ½x½, while MERRA-2 has a resolution 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.