expected vs. actual PV Watts

I am attempting to use SAM to compare expected output from a solar array to actual production. The array has a weather station located on site, so I am able to get total radiation values from the weather station. I am trying to pass this into the C# wrapper for the SCC module "pvwattsfunc" to get the snapshot value of the expected ac power.

For the snapshot time I am attempting to test with, the calculated poa value comes back as 251.3582 W/m2, which seems reasonable given the measured sol value of 263.4 W/m2 from the weather station. The expected ac power, however, returns as 167,139.1 W vs the measured output from the array at that time of 45,500 W. The total system nameplate capacity is 858kW.

I don't have reason to believe that the array is actually performing that poorly, but rather than since I'm new to PVWatts and SAM that I am doing something wrong with my input parameters. I put the weather station solar value into both the current beam and the previous step poa value, although I don't think that the previous step should matter since I'm just taking one value at a time.

Any suggestions or recommendations would be appreciated. My input parameters from code are listed below for reference.

Thanks in advance,

Mikkel

One correction - my values were low because I had thought the production value was already ac watts, when in fact it was Wh for a 15 minute period. Of course, that means that now my values are generally higher than predicted by the SAM module.

One question I still have is if there is an explanation as to why the poa value differs so significantly from the measured radiation value. Should I be putting my whole radiation value in to the beam radiation parameter? The actual break out between beam and diffuse will obviously vary quite a bit depending on cloud cover, etc., but the weather station I get data from does not break out the components of the radiation.

Paul,

I was able to incorporate the DISC model that you recommended into my code to break out DNI and DHI values from my GHI measurement. For the most part, I think it is working well, but I see some anomalies as I get to the end of the day with the first day's data that I'm testing with. Specifically, I get quite a few POA values that are zero when the measured GHI is still higher than zero. In one time step, the POA value actually goes negative - which I don't think should be possible. The DNI and DHI values passed in for that timestep are both positive, so I'm a little bit at a loss as to why the POA would suddenly drop below zero, then come back again in the next step.

Any ideas as to what may be going on there? I have attached a spreadsheet called 'TestData20130927.xlsx' to the original message that shows the output from the time span in question.

Thanks in advance,

Mikkel