UV intensity calculation

I am using the different models of UV system (Make-Alfa UV), i.e. API-20, API-35, API-45, API-45 R1 etc… How can i calculate the minimum required intensity in Watts/Square Meter?

It s required to study for deciding the minimum intensity requirement for microbial load of water.
Generally minimum intensity > 35 watt/m2 should be maintain for within limit of microbial load.

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