Terms & Concepts

Irradiance and weather data are produced using a variety of different measurement and modeling techniques. Knowing the source of each field will allow you to better apply the data within your application. This section explains some of the data fields you might come across to make working with the API easier.

Irradiance Data

Global Horizontal Irradiance (GHI)

Global horizontal irradiance (GHI) is the measure of the total radiation that is incident on a plane horizontal to the surface of the earth. The units of GHI are in watts per meter squared. GHI is the sum of the direct normal (DNI) and diffuse (DIF) irradiance conditions. The direct normal irradiance component is adjusted by the geometric relationship of the sun's position in the sky relative to location of interest.

The Irradiance and Weather Data API uses SolarAnywhere data as its source for GHI. SolarAnywhere generates GHI measurements from 1998 through the present, and forecast data out to 90-days in the forecast horizon. SolarAnywhere generates these measurements by using the latest and most advanced SUNY satellite-to-solar irradiance model developed in collaboration with Dr. Richard Perez from The State University of New York at Albany (SUNY-Albany).

Direct Normal Irradiance (DNI)

Direct normal irradiance (DNI) is the measure of the solar radiation perpendicular (normal) to the sun. The units of DNI are in watts per meter squared. DNI can also be referred to as the direct beam radiation.

The Irradiance and Weather API uses SolarAnywhere data as the source for DNI. SolarAnywhere generates DNI measurements from 1998 through the present, and forecast data out to 90-days in the forecast horizon. SolarAnywhere generates these measurements using the latest and most advanced SUNY satellite-to-solar irradiance model developed in collaboration with Dr. Richard Perez from The State University of New York at Albany (SUNY-Albany). SolarAnywhere DNI data is produced using Dr. Perez's DIR_INT model.

Diffuse Irradiance (DIF)

Diffuse irradiance (DIF) is the measure of the solar radiation that is incident on a horizontal surface to the earth, created by the sky. DIF contains scattered direct light from the sun. The units on DIF are in watts per meter squared.

The Irradiance and Weather API uses SolarAnywhere data as the source for DNI. SolarAnywhere generates DNI measurements from 1998 through the present, and forecast data out to 90-days in the forecast horizon. SolarAnywhere generates these measurements using the latest and most advanced SUNY satellite-to-solar irradiance model developed in collaboration with Dr. Richard Perez from The State University of New York at Albany (SUNY-Albany). SolarAnywhere DIF data is produced by enforcing the geometric relationship between GHI and DNI, where DIF = GHI - DNI*cos(Z) and Z = the sun's zenith angle.

Additional Weather Data

Wind Speed

Wind speed is a measurement of the flow of air, typically caused by pressure differentials in the atmosphere. For PV applications, wind speed data can be used to assess cooling factors, which ultimately impact temperature derate and PV module efficiency.

Data is available in all geographic regions where irradiance data is present in time-series, forecast and typical-year data types. Wind speed data is sourced from multiple forecast and reanalysis numerical weather models, and ranges in spatial resolution from 10-km down to 5-km grids. Wind speed data is predicted at 10-meters off of the surface. Wind speed data is in units of meters per second.

Dry Bulb Temperature

Dry bulb temperature is the measurement of the ambient air temperature with account of irradiance or moisture. For PV applications, the dry bulb temperature data is an input into the calculation of the module derate due to temperature. Higher temperatures cause the PV modules to produce less power for equivalent levels of incident irradiance.

Data is available in all geographic regions where irradiance data is present in time-series, forecast and typical-year data types. Temperature data is sourced from multiple forecast and reanalysis numerical weather models, and ranges in spatial resolution from 10-km down to 5-km grids. Temperature is predicted at 2-meters off of the surface and has units of Celsius.

Relative Humidity

Relative humidity (RH) is the measurement of air moisture content as a ratio of the partial pressure of water to the vapor pressure at ambient temperature. For PV applications, RH can be used to better predict the energy output of PV modules through spectral impacts on the incident irradiance.

Data is available in all geographic regions where irradiance data is present, currently in time-series and typical-year data types. Forecast RH is not currently available. RH data is sourced from multiple reanalysis numerical weather models, and ranges in spatial resolution from 10-km down to 5-km grids. Relative humidity is in percentage units.

Precipitation

Precipitation is a measurement of the volume of liquid or solid water (primarily) that condenses out of the atmosphere. For PV applications, precipitation information can help determine the operational performance of a PV system and help identify certain conditions such as soiling.

Data is available in the Continental U.S. in the time-series data type. Forecast and typical-year precipitation is currently not available via the API. Precipitation is broken into two forms: solid and liquid precipitation. Each is denoted by a separate field, SolidPrecipitation and LiquidPrecipitation respectively, via the API.

Precipitation is sourced from reanalysis models, and ranges in spatial resolution from 5-km down to 1-km. Both solid and liquid precipitation are in units of kilogram per meter squared. Daily totals are down sampled to the temporal resolution specified by the client.

Snow Depth

Snow depth is the measurement of the height of solid precipitation accumulated on the ground. For PV applications, snow depth information can help determine the operational performance of a PV system, and help identify certain conditions such as snow accumulation as a cause of under-performance.

Data is available in the Continental U.S. in the time-series data type. Forecast and typical-year precipitation is currently not available via the API. Snow depth is sourced from reanalysis models, and ranges in spatial resolution from 5-km down to 1-km. Snow depth is in units of meters. Daily totals are down sampled to the temporal resolution specified by the client.

What’s Next?