0.1% accuracy over 1000:1 means that the particular measurement of the Energy Metering IC has a typical accuracy of 0.1% over the range of current signals from the full-scale reading to as low as 1000 times smaller than that. For Example, table 2 in the ADE7878 datasheet shows that the accuracy of total active power is 0.1% over a dynamic range of 1000:1, which means that even if the input is reduced to 0.5 mV, the measurement of total active power does not have an error of more than 0.1% (The full-scale input is 500mV). It is to be remembered that the accuracy specification is typical, as it is a result of several other external measurement factors that have to be quite accurate as well. Note that the accuracy measurement is done on a system level, after calibrating the chip.
This FAQ was generated from the following discussion: Accuracy of ADE ICs