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Old 08-02-16, 11:26 AM   #541
stevehull
Steve Hull
 
Join Date: Dec 2012
Location: hilly, tree covered Arcadia, OK USA
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Pinball (or others with years of PV production data),

Since you have over five years with your array, you can perhaps answer/comment on my summer/winter PV array kWhr production question.

I (like most of us) use PVWatts to give me the effective number of sun hours per day for my lat/long., array pitch and direction. Knowing the total max kW of the system, this allows me to calculate a predicted number of kWhrs for the month.

But PVWatts does not account for varying outside daytime temperatures (by month) and all panels have a negative correlation coefficient for output voltage. The STC is based on a 20 degree C base and temps above that decrease power and energy output and colder temps increase it. The PTC value (generally about 90% of STC) kinda sorta includes temperature.

PVWatts simply assumes more sun in summer and less in winter with no inputs for summer/winter temperature changes.

For my predictive spreadsheet, I have used a -12% decrease for monthly July kWhr output and a +10% increase for January with intermediate coefficients in between for other months.

Without this temperature modification, my predicted monthly kWhr output was lower in summer months and higher in winter months compared to what PVWatts predicts.

I just finished designing another client's home (all Enphase installs) and adding my + and - temperature coefficients really closes in on a better monthly kWhr prediction.

Are you aware of a better predictive algorithm (one that includes historic monthly daily average temperature) than PVwatts?

Thoughts?


Steve

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