The difference between “international” and “absolute” electrical units is explained, and the reason for the change made in 1948 from one system to the other is given. The uncertainty of the electrical units defined in terms of length, mass and time is estimated to be about 20 parts in a million, although the accuracy of comparison of electrical quantities may be as high as one part in ten million. The loss of accuracy in the measurement of voltage, current and power when transferring from direct to alternating current is explained: it is estimated that these quantities cannot be measured at power frequencies to better than one part in 10000.