Accuracy of electron counting using a 7-junction electron pump

We have operated a 7-junction electron pump as an electron counter with an error per pumped electron of 15 parts in 109 and an average hold time of 600 s. The accuracy and hold time are sufficient to enable a new fundamental standard of capacitance. We compare the measured accuracy of the pump as a function of pumping speed and temperature with theoretical predictions based on a model which includes stray capacitance.