Question on how SAM was used in an NREL report on rooftop PV potential in the US

Hello,

I'm writing with reference to an NREL report attached here -  "Rooftop Solar Photovoltaic Technical Potential in the United States".

Specifically on Page 9, there's a section that mentioned: "For each city, SAM was used to calculate the number of hours a rooftop would need to be in sunlight to produce 80% of the energy produced by an unshaded system of the same orientation."

May I know how this calculation was done in SAM please? I've tried to reach out to the project team that authored the report too, but I thought I'd put this question on the SAM User Forum as well as it is related to the tool specifically.

Thanks a bunch!

Hi Tan,

I have replaced your attachment with a link to the original document: www.nrel.gov/docs/fy16osti/65298.pdf .

I was not involved in the analysis described in the document. The description you quote, it sounds like the authors calculated the number of peak sun hours from SAM's annual production estimate (or 80% of the estimate). Here is an article describing how to calculate peak sun hours: www.pveducation.org/pvcdrom/properties-of-sunlight/average-solar-radiation

Best regards,
Paul.

Hi Paul,

Thanks for looking into my query.

For the example city of Spokane in Washington State, SAM returned an annual value of 3.81kWh/m2/day for the Global horizontal insolation (i.e. 3.81 sun-hours/day),

The methodology as documented by NREL report then lists a dimensionless value of 0.3608 (% needed for 80% generation) that was supposedly calculated using SAM.  Do you know how I might derive this value of 0.3608?

This value I'm referring to above is highlighted in the NREL document attached here (under Column I).

Would greatly appreciate your advice.

Best,
Congyi

Hi Congyi,

A couple of things to consider:

• The weather file you are using for Spokane may be different from the one used by the study authors, so the annual average GHI may be different.
• They may have based their calculations on the plane-of-array (POA) irradiance rather than the GHI. SAM reports POA irradiance in the results.

Again, I was not involved in the study, so am not in a position to explain their methodology.Best regards,
Paul.

Thanks for your advice Paul.