A few short notes; maybe of some help for shortcut calculations ― The actual heat transferred to the air current is limited by its capacity to uptake heat, both as sensible and latent heat. For the air current, it is important to consider both the dry-bulb and the wet-bulb temperature. The last depends on the air relative humidity. For higher relative humidity, there would be a lower capacity of the air to uptake latent heat, compared with dry air, if both are considered at the same dry-bulb temperature. The air wet-bulb temperature may possibly approach reasonably well that of the wet (water evaporating) glass surface, when the water flow and the solar radiation heat contribution are both relatively small, regardless of the feed water temperature. After the heat balance, including radiation ― not converted to electrical power ― and sensible heat contributions, the latent heat increment for the air current can be inferred, from which the evaporated water can be accounted for. 

experimentally it is an easy task

if you have a closed cycle of water flow measure the initial mass of water in the supplying tank after some time measure again the difference is the water lost via evaporation

https://www.researchgate.net/publication/283050782_EXPERIMENTAL_AND_NUMERICAL_INVESTIGATION_OF_PHOTO-VOLTAIC_MODULE_PERFORMANCE_VIA_CONTINUOUS_AND_INTERMITTENT_WATER_COOLING_TECHNIQUES_Experimental_and_Numerical_Investigation_of_Photo-Voltaic_Module_P?_iepl%5BviewId%5D=7NrHiLKu9rEOMRAocwUFvPiP&_iepl%5BprofilePublicationItemVariant%5D=default&_iepl%5Bcontexts%5D%5B0%5D=prfpi&_iepl%5BinteractionType%5D=publicationTitle

Article EXPERIMENTAL AND NUMERICAL INVESTIGATION OF PHOTO-VOLTAIC MO...

Dear Colleague, 

The questions is very important, but there are some data must be given like if the system is in a closed or open loop, etc.

Regards!

Dr. Carlos,

Thank you for your response, have messaged you for seeking further details and clarifications to the question.

Best Regards!

Mr. Akram,

Thank you for your response, have messaged you for seeking further details and clarifications to the question.

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Dr. Adel,

Thank you for your response, have messaged you about further details and clarifications to the question.

Best Regards!

Hi sir

According to formual you can assume a two phase flow. one streaming of air and another streaming of water above photocell and you should consider the rate of evaporation between two phases.

Dr. Mehdi,

Thank you for your kind response, have messaged you for further details that I am looking for towards this.

Best Regards,