1. A method for calculating an RMS value of an AC signal, the method comprising: receiving a plurality of data points from an analog-to-digital converter, wherein the analog-to-digital converter is comprised in a measurement system, wherein the plurality of data points comprise at least a part of the AC signal; calculating an interpolated window point from window data; calculating intermediate results using the interpolated window point and a data point from a first subset of the plurality of data points; and calculating an AC RMS value for the plurality of data points.