A simple, physically transparent method is developed to calculate the electric charge per neutron captured in prompt response self-powered neutron detectors (SPNDs), which contributes to the emitter-collector current. This charge is written as the energy integral of the product of two functions: the spectral function S(E), which is the energy spectrum of all electrons resulting from prompt gamma interactions with the atoms of the emitter, and the electron spectral contribution ∈ f(E), which is the probability that an electron released with energy E reaches the collector. The function ∈ f(E) is given an analytical approximate expression derived from an analytical approximation obtained for the path length probability distribution function. The exact expression of ∈ f(E) is also obtained analytically for infinitely long emitters in terms of special functions. The method allowed the creation of an extremely fast algorithm to calculate the effective charge and was applied to cobalt prompt response SPNDs.