The amount of charge transferred to ground during long continuing currents in natural downward flashes can be obtained either through direct current measurements or using remote electromagnetic fields, if direct measurements are not feasible. In this study, measurements of a single-station E-field antenna were used to estimate charge transfer during continuing currents. With time-synchronized high-speed video recordings serving as ground-truth data for continuing current occurrence, we estimate transferred charge during long continuing currents in 140 natural downward flashes from electric field changes, assuming a simple monopole charge model. We present average parameters for the duration, transferred charge, amplitude, and the average amplitude on five segments along the channel. Further, we perform a simulation to investigate the estimation performance of the monopole charge model, when the cloud charge is spatially extended. Using an extended charge distribution model, we generate single-station remote field waveforms and infer the charge with the monopole model. Comparison of the known and inferred charge yields estimation errors depending on the leader orientation, its spatial extension, and the observation distance. A Monte Carlo approach is carried out to statistically evaluate parameter deviations. This novel estimation error analysis sheds light on the limitations of the applied simple techniques for charge transfer estimation of long continuing currents using single-station electric field measurements.