Using measured solar irradiation in EnergyPlus

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Not directly. SurfaceProperty:LocalEnvironment allows you to specify the External Shading Fraction Schedule for a surface. To use the measured solar data, run a preliminary simulation with ShadowCalculation, External Shading Calculation Method=InternalCalculation (default) and output these output variables:

Output:Variable,*,Surface Outside Face Sunlit Fraction,hourly;
Output:Variable,*,Surface Outside Face Incident Solar Radiation Rate per Area,hourly;
Output:Variable,*,Surface Outside Face Incident Beam Solar Radiation Rate per Area,hourly;
Output:Variable,*,Surface Outside Face Incident Sky Diffuse Solar Radiation Rate per Area,hourly;
Output:Variable,*,Surface Outside Face Incident Ground Diffuse Solar Radiation Rate per Area,hourly;

Report these hourly or at timestep depending on the frequency of the measured data. Then calculate the sunlit fraction implied by the measured data.

MeasuredSunlitFrac = (MeasuredSolar - EnergyPlusSkyDiffuse - EnergyPlusGroundDiffuse)/[(EnergyPlusBeam/EnergyPlusSunlitFrac)]

(I think that's right). Basically, EnergyPlus is going to apply the same sky diffuse and ground diffuse, so you want to calculate a sunlit fraction that alters the beam to match the measured total.

Then run a final simulation using ShadowCalculation, External Shading Calculation Method=ScheduledShading, using SurfaceProperty:LocalEnvironment to apply the custom sunlit fractions (note that the field name is "external shading fraction but it's really used as sunlit fraction). Also, ScheduledShading is global, so when this is active, any surface without a SurfaceProperty:LocalEnvironment object will be assumed fully sunlit.