Solar journey: Design and predicting annual production. Part 2

Tesla's plan estimated annual production at 6622 kwh. I have not designed a system before nor had one. It is close to what other quotes called out. Aside from waiting a  year to see, is there a way to run the numbers your self?

The answer is yes. The National Renewable Energy Lab (NREL) has a user friendly site an calculator to help you do this. Its called PVWatts. Link below:

https://pvwatts.nrel.gov/

PVWatts requires your location so it can use its database to determine the seasonal average solar radiation (accounting for weather).

PV Watts , how to use , Colorado solar array power output
Once you enter.your location, it is on to.entering the PV  system properties

If you have all of your panels in one direction, this easy. In my case, the panels are split between East and South....so I have to enter each half separately, then add them together to get the total  system estimate.

PVWatts provides typical default numbers. Here is what I entered using my Southern string as an example:
-Half system size=2.78 kw (7 panels)
-Module type= premium (~19% efficiency)
-Array type: roof mount
-System losses: 14% (used default, Tesla did not provide)
-Tilt: 40deg (this is roof pitch or angle, Tesla estimated 30 deg incorrectly in their plan)
Azimuth: 183 deg (I used the Tesla number. 180deg would be due South)

Under Advanced, my Delta inverter is specced at 97.5% efficient.

Once you have that you are done. You now habe a monthly breakdown of solar radiation, PV system energy output, and $ value if you input your electrical rate.

PVWatt vs Tesla Estimate
I did this a few times for both the East and South strings. Here is my total PVWatt production estimate compared to Tesla's plan.

PVWatt.                   Tesla
6,875 kwh               6,622 kwh

That is about a 4% difference. However, if you look closely, PVWatts gives you range and a confidence interval. In fact, you can also get a daily and hourly breakdown for the whole calender year.

Taking the high and low values, Tesla's production estimate is quite close to the middle of that.
Conclusion on the Tesla estimate:
It is perfectly reasonable estimate and agrees with the NREL estimate.

#solar #Tesla #review #NREL #PVWatts

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