Hello,
The POA and cell temperature variables are "inout" variables. As inputs, they are passed to the cell temperature model as starting values for the algorithm. As outputs they are the POA calculated from the beam and diffuse inputs, and the cell temperature resulting from the cell temperature model. So you are not overriding the calculated POA or cell temperature outputs.
For the first time step of the simulation, the POA and cell temp variables are the initial conditions of the module at the beginning of the year (the values are probably not too important). For subsequent calls to the pvwatts5_1ts module, the values should be set to the last calculated values from the previous time step because the module temperature model in PVWatts accounts for transient heating/cooling of modules.
Best regards,
Paul.