How EnergyPlus calculates the heat transferred to the construction by an internal source is documented in the first part of the Low Temperature Radiant System Model Encyclopedic Reference if the Engineering Reference (see the subsection: 'Extension of Time Series Solutions to Include Heat Sources and Obtain Internal Temperatures').

The 'heat exchanger formulation' describes how the heat transferred at the radiant temperature source effects the plant loop (and associated HVAC simulation) in a hydronic system. There's no need for a corresponding formulation for the electric system, since the heat from an electric system is easily related to the current passing through the heated wire, which is obtained from the input value 'Maximum Electric Power to Panel' and the system control.