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This question is being submitted on behalf of a user:

I'm building an emergency solar generator and am trying to understand your tables. Getting the solar Peak Solar Hours (PSH) values correct is critical to my calculations. This is going to be long-winded, so I apologize in advance. After using this website: https://eosweb.larc.nasa.gov/cgi-bin/sse/grid.cgi and then selecting "parameters for tilted solar panels,” I see two options: “Minimum Radiation Incident on An Equator-pointed Tilted Surface” and "Maximum Radiation Incident on An Equator-pointed Tilted Surface." My assumptions are that these options show the lowest AVG and highest AVG PEAK SOLAR HOUR (PSH) values for a particular month in a particular location measured over several years of observation.

Here is my definition of PSH: Peak Sun Hours (PSH) is a measure of the available daily solar resource. PSH are also known as ‘insolation’ and depend on the specific geographic location. PSH are measured in hours per day. Actually, it is the number of hours of Global Average Solar Radiation (GASR) [Wh/m2/day] normalized with respect to 1,000W/m2 of irradiance, or PSH=GASR/1000.

Are my assumptions correct up to this point? I also assume that the data in the OPT row is the best one can expect for that particular scenario (highest or lowest average) and OPT ANG was the angle that gave those particular results.

Example 1: For location, Latitude: +30.33 Longitude: -81.547, selecting “MINIMUM Radiation Incident on An Equator-pointed Tilted Surface,” I see 2.79 in the January column for the OPT value at an OPT ANG of 47.0 degrees. If my above assumptions are correct, this means that the lowest average radiation measured for the month of January over several years of measuring was 2.79 and the optimal angle for collecting the most energy for that month was 47.0 degrees. It must have been a cloudy / rainy month.

Example 2: For location, Latitude: +30.33 Longitude: -81.547, selecting “MAXIMUM Radiation Incident on An Equator-pointed Tilted Surface,” I see 5.17 in the January column for the OPT value at an OPT ANG of 54.0 degrees. If my above assumptions are correct, this means that the highest average radiation measured for the month of January over several years of measuring was 5.17 and the optimal angle for collecting the most energy for that month was 54.0 degrees. I am guessing it was a cloud-free month.

Are my assumptions correct so far?

One final question: for the above two examples, we see that the OPT ANG varied from 47.0 degrees to 54.0 degrees. The Earth’s wobble is a 26K year cycle so it can’t be that. Can you explain the reason for this difference?

Thank you for your assistance in this matter.

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  1. Your interpretation of OPT ANG and OPT is correct, however I should note that the solar radiation in the SSE table is not directly measured but based upon inferred from satellite observations.  Also, the monthly and yearly values are averaged values over a 22 year period from July 1, 1983 through June 30, 2005.  As you point out, the Earth's wobble is on a 26,000 year cycle so it's affect would be negligible over the 22 year averaging period