energy measured(kWh)/(Irradiance(kWh/m2) on the panel x Active area of PV module(m2) x PV module efficiency)...For the incident solar irradiation you determine that using a measuring gage,while this calculation is done over a period of time,optimally(1 year) however you could choose to select shorter periods.

Dear Erika Carmen:

You can benefit from this valuable articles about your topic:

@https://www.pvsyst.com/help/performance_ratio.htm

@https://www.scirp.org/journal/paperinformation.aspx?paperid=85611

@https://pvsol-online.valentin-software.com/

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Also this valuable online calculator:

@https://pvwatts.nrel.gov/

Also these valuable attached pdf files...

Best wishes...