A Radioisotope Thermophotovoltaic (RTPV) Cell is a device used to convert heat energy into electrical energy. The electric generation capacity of RTPV cell depends on the radiative heat transfer between its two surfaces: the emitter surface heated by radioisotope thermal source and the receiving photovoltaic (PV) cell surface. The spectral directional surface properties and the surface temperatures of emitter and PV cell surface play important roles in quantifying the radiative heat flux of RTPV cell. This paper establishes the required analytic flat plate solutions to calculate the radiative heat flux of RTPV cell. The results obtained using the analytic solutions developed in this study have been qualitatively validated with the results of numerical simulations performed by a commercially available software. The effect of the surface temperatures and emitter surface coating on RTPV cell capacity is also studied and analyzed by both the methods. The results obtained from both the methods show that PV cell surface temperature has negligible effect on RTPV cell capacity as compared to the emitter surface temperature. Also, the radiative heat flux of RTPV cell with coated emitter is found to be significantly higher than that of RTPV cell with uncoated emitter surface. The analytical methods can be used to estimate the net radiative heat flux of RTPV cell for different surface temperatures and are independent of the dimensions of RTPV cell.