# ERA-Interim **URL:** \ **Category:** Other - Datasets and Usage\ **Tags:** c3s, ecmwf\_reanalysis, era-interim\ **Created:** [1 July 2019 12:15 UTC](https://forum.ecmwf.int/t/era-interim/1304 "2019-07-01T12:15:05Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![DIMITRIOS\_ZARAGKAS](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/d/9e8a1a/32.png) [@DIMITRIOS\_ZARAGKAS](https://forum.ecmwf.int/u/DIMITRIOS_ZARAGKAS)\ **Post date:** [1 July 2019 12:15 UTC](https://forum.ecmwf.int/t/era-interim/1304/1 "2019-07-01T12:15:05Z")
ERA-Interim SSRD Are Surface solar radiation downwards and Global Horizontal Irradiance identical physical quantities in order to study about photovoltaics systems? ---
**Author:** ![Kevin\_Marsh](https://forum.ecmwf.int/user_avatar/forum.ecmwf.int/kevin_marsh/32/368_2.png) [@Kevin\_Marsh](https://forum.ecmwf.int/u/Kevin_Marsh)\ **Post date:** [1 July 2019 12:30 UTC](https://forum.ecmwf.int/t/era-interim/1304/2 "2019-07-01T12:30:13Z")
Hi [DIMITRIOS ZARAGKAS](https://confluence.ecmwf.int/display/~zaragkas@hotmail.com) This forum is for C3S announcements rather than technical questions, so I will move it to the CDS General User Forum, Thanks, Kevin C3S User Support ---
**Author:** ![Natalia\_Odnoletkova](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/n/58f4c7/32.png) [@Natalia\_Odnoletkova](https://forum.ecmwf.int/u/Natalia_Odnoletkova)\ **Post date:** [19 July 2019 10:03 UTC](https://forum.ecmwf.int/t/era-interim/1304/3 "2019-07-19T10:03:36Z")
Hi, Dimitrios! I am also using ERA5 for the purpose of estimation solar irradiance and PV panel output . Per my opinion, yes. These are identical quantities. This is from ERA5-Land documentation: "To a reasonably good approximation, this variable (Surface solar radiation downwards) is the model equivalent of what would be measured by a pyranometer (an instrument used for measuring solar radiation) at the surface". Pyranometer which is placed horizontally measures GHI. ---
**Author:** ![DIMITRIOS\_ZARAGKAS](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/d/9e8a1a/32.png) [@DIMITRIOS\_ZARAGKAS](https://forum.ecmwf.int/u/DIMITRIOS_ZARAGKAS)\ **Post date:** [19 July 2019 10:48 UTC](https://forum.ecmwf.int/t/era-interim/1304/4 "2019-07-19T10:48:14Z")
Hi, Natalia! First of all thank you for your reply. My main research is about available solar irradiance in an specific area but it would be useful for me to find an equation to calculate a PV panel output. Could you suggest me a paper to find something relevant ? I made my research about it but I want to be sure that I follow the right procceess.  Thank you in advance. ---
**Author:** ![Natalia\_Odnoletkova](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/n/58f4c7/32.png) [@Natalia\_Odnoletkova](https://forum.ecmwf.int/u/Natalia_Odnoletkova)\ **Post date:** [19 July 2019 11:53 UTC](https://forum.ecmwf.int/t/era-interim/1304/5 "2019-07-19T11:53:47Z")
Dear  Dimitrios, Thank you for your question. The calculation of PV panel output is not straightforward. First of all, I estimate radiation incident on the panel. I followed ASHRAE handbook fundamentals (2017) "Calculation of Clear-Sky Solar Irradiance Incident On Receiving Surface".  For this, direct and diffuse components of incident on panel irradiance should be calculated, in the Handbook you can find the procedure. In short , solar elevation angle is important to obtain. For instance, it allows to convert horizontal direct irradiance (total sky values I took from ERA5) to direct normal (measured perpendicular to the sun rays). You can find all formulas at chapter "CALCULATING CLEAR-SKY SOLAR RADIATION".  When incident radiation is calculated, output may be estimated if efficiency of the panel is known. However, I am sure there are other ways to estimate panel performance , I just described the method tat I used. I found ASHRAE procedure very clear, easy to follow, and credible. Moreover, they update their documentation according to recent developments in the field.  ---
**Author:** ![DIMITRIOS\_ZARAGKAS](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/d/9e8a1a/32.png) [@DIMITRIOS\_ZARAGKAS](https://forum.ecmwf.int/u/DIMITRIOS_ZARAGKAS)\ **Post date:** [19 July 2019 13:10 UTC](https://forum.ecmwf.int/t/era-interim/1304/6 "2019-07-19T13:10:26Z")
Dear Natalia  I am a little bit confused. Which indicator is more appropriate to study about the incoming solar power in an area in order to settle a PV panel? Is the Surface Solar Radiation Downwards  or Surface net Solar Radiation for Clear-skies? As I have studied so far I thought that Surface Solar Radiation Downwards is more appropriate because includes direct and diffused parts of incident Irradiance. Is it right? ---
**Author:** ![Natalia\_Odnoletkova](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/n/58f4c7/32.png) [@Natalia\_Odnoletkova](https://forum.ecmwf.int/u/Natalia_Odnoletkova)\ **Post date:** [20 July 2019 09:24 UTC](https://forum.ecmwf.int/t/era-interim/1304/7 "2019-07-20T09:24:30Z")
Dear Dimitrios, Sorry for confusion in my comment , because I referred to Clear Sky model. You are correct, Surface Solar Radiation Downwards is more appropriate. I referred to ASHRAE because there are all required formulas for calculation of solar angle and incident radiation on tilted surface. The thing that I did in my calculations: I followed ashrae "CALCULATING CLEAR-SKY SOLAR RADIATION" chapter, but I used Total sky direct solar radiation at surface from era5 instead of clear sky radiation, and diffuse solar radiation at surface from era5 as well which is Surface Solar Radiation Downwards minus total sky direct solar radiation.