# Tilted Surfaces

For a data request of "Solar Irradiance for Equator Facing Tilted Surfaces", via the POWER API the following are computed; solar irradiance for the angle equal to the absolute latitude of the location, ±15° of the absolute value of latitude, horizontal, vertical, and optimally tilted panels. The optimal tilt angle for a given month is the tilt angle when the monthly average irradiance for the month attains the maximum.

POWER provides the solar irradiances for Equator facing set of tilted surfaces with the functions above applied at a specific point location.

### Input Parameters¶

The POWER API computes the set of surfaces using the same climatological monthly average. The POWER API recursively runs through the months and required parameters to create the final output using the following inputs.

Name Standard Minimum Maximum
ghi ALLSKY_SFC_SW_DWN ALLSKY_SFC_SW_DWN_MIN ALLSKY_SFC_SW_DWN_MAX
dhi ALLSKY_SFC_SW_DIFF ALLSKY_SFC_SW_DIFF_MAX ALLSKY_SFC_SW_DIFF_MIN
alb ALLSKY_SRF_ALB ALLSKY_SRF_ALB ALLSKY_SRF_ALB
dec SG_DEC SG_DEC SG_DEC

### Output Parameters¶

The surfaces are computed by the Tilt Value Equation that creates a solar irradiance value for a specific point location and tilt angle. This equation is called by the optimal value equation that is used in producing the set of surfaces. The surfaces are angle equal to the latitude of the location and plus, minus 15 degrees of the latitude in addition to horizontal and vertical panes.

Irradiances are computed for surfaces tilted toward the Equator at the following angles:

1. horizontal panel (tilt angle = 0°)
2. tilt angle equal to abs(latitude)
3. tilt angle equal to abs(latitude)+15°
4. tilt angle equal to abs(latitude)-15°
5. vertical panel (tilt angle = 90°)

Below is the table of the individual parameters computed:

Abbreviation Long Name
SI_EF_TILTED_SURFACE_HORIZONTAL Solar Irradiance for Equator Facing Horizontal Surface
SI_EF_TILTED_SURFACE_LAT_MINUS15 Solar Irradiance for Equator Facing Latitude Minus 15 Tilt
SI_EF_TILTED_SURFACE_LATITUDE Solar Irradiance for Equator Facing Latitude Tilt
SI_EF_TILTED_SURFACE_LAT_PLUS15 Solar Irradiance for Equator Facing Latitude Plus 15 Tilt
SI_EF_TILTED_SURFACE_VERTICAL Solar Irradiance for Equator Facing Vertical Surface
SI_EF_OPTIMAL Solar Irradiance at the Optimal Angle
SI_EF_OPTIMAL_ANG Solar Irradiance Optimal Angle in Degrees
SI_EF_OPTIMAL_ANG_ORT Solar Irradiance Tilted Surface Orientation listed as a N/S Orientation

These computed parameters cannot be requested individually

• When using the Data Access Viewer (DAV) they are available in single point data access widget with the climatology temporal level selected and Solar Irradiance for Equator Facing Tilted Surfaces (Set of Surfaces) in the group Tilted Solar Panels.
• When using the POWER API directly as single point with the climatology temporal level just include the SI_EF_TILTED_SURFACE parameter abbreviation in the parameters list.

Notes

• If an input parameter includes a NaN (-999 fill value) the POWER API will not compute that location.
• The solar irradiance values are returned in the community specific units conventionally used in the solar energy community.

### Validation¶

Comparison Plots: POWER Tilted Surface to BSRN Data

The comparison plots for the comparison of POWER tilted surface with the BSRN data for Apr. 2000 to Dec. 2020.

• Left — Scatter Plot:
• Right — Histogram of Differences: