Solar radiation threshold in PVWatts

Does PVWatts use some sort of thresholds value for solar radiation? Meaning is there a minimal solar radiation value which generates an hourly AC energy output (all other solar radiation values below that one generate 0 kWh per hour)? Or even the lowest solar radiation value is taken into account and used to calculate the AC energy output?

Thank you for the reply.

Hello,

There is not a minimum incident solar irradiance threshold for SAM's implementation of the PVWatts model. Are you observing results that suggest that there is?

If you include losses in your analysis, at low irradiance levels, the losses might be greater than the calculated DC output, which might make it appear that there is a minimum threshold.

Best regards,
Paul.



Best regards,
Paul.

Thank you for the reply Paul.

You are probably right about the losses part, I haven't thought about it.
I can not check it though, there seems to be some issues with PVWatts website, for the last couple of days. The message "Loading Map of Weather Stations" just hangs on indefinitely, on the "Solar recurse data" page.

Hello,

The losses I mention are part of PVWatts in SAM, not the web version of PVWatts.

The weather data service for web version of PVWatts does seem to be down at the moment. Please try again later -- we are working to fix the problem.

Best regards,
Paul.

Hi Paul,

I am using SAM 2013.1.15 (that's the last version which supports my Windows XP).

This version of SAM has the same DC to AC Derate Factor system input, your online PVWatts version has.

Did I misunderstand what you meant by "losses"?

In older versions of SAM, the losses are defined on the Performance Adjustment input page. However, these losses are expressed as a fraction of the system's output, so they would not cause the appearance of a threshold as I suggested above.
Older versions of SAM use a different version of PVWatts than the current version (PVWatts V1 instead of PVWatts V5). As described in Section 10 of the PVWatts Technical Reference , PVWatts V1 applied a correction to the array output at low light levels that caused the output to go to zero under those conditions. That correction was removed in PVWatts V5, so you won't see that effect in the web version of PVWatts or in the current version of SAM.
Best regards,

Paul.