A method of identifying a meter that is out of calibration comprising acts of: obtaining a power measurement value for each of a plurality of metering devices (A0, B0, B1, B2, B3, C0, C1, C2, C3, C4, C5, C6) arranged in a hierarchy (202, 204, 206), the hierarchy including a plurality of levels with at least one of the plurality of metering devices in each of the plurality of levels; selecting a candidate metering device from one of the plurality of levels of the plurality of metering devices; calculating a plurality of comparison values for the candidate metering device based on the power measurement values of at least two metering devices of the plurality of metering devices excluding the candidate metering device, wherein at least one of the at least two metering devices is located in a different level in the hierarchy than the candidate metering device and the plurality of comparison values is calculated based on at least two different sets of one or more metering devices of the plurality of metering devices; calculating a difference value between the power measurement value of the candidate metering device and a first comparison value of the plurality of comparison values; and identifying the candidate metering device as being out of calibration based on the difference value; the method being characterised in that identifying the candidate metering device as being out of calibration based on the difference value includes calculating an additional difference value between the power measurement value of the candidate metering device and an additional comparison value of the plurality of comparison values, and determining if the difference value and the additional difference value are positive or negative; and identifying an increased likelihood of the candidate metering device being out of calibration in response to determining the difference value and the additional difference value are both positive or both negative. obtaining a power measurement value for each of a plurality of metering devices (A0, B0, B1, B2, B3, C0, C1, C2, C3, C4, C5, C6) arranged in a hierarchy (202, 204, 206), the hierarchy including a plurality of levels with at least one of the plurality of metering devices in each of the plurality of levels; selecting a candidate metering device from one of the plurality of levels of the plurality of metering devices; calculating a plurality of comparison values for the candidate metering device based on the power measurement values of at least two metering devices of the plurality of metering devices excluding the candidate metering device, wherein at least one of the at least two metering devices is located in a different level in the hierarchy than the candidate metering device and the plurality of comparison values is calculated based on at least two different sets of one or more metering devices of the plurality of metering devices; calculating a difference value between the power measurement value of the candidate metering device and a first comparison value of the plurality of comparison values; and identifying the candidate metering device as being out of calibration based on the difference value; the method being characterised in that identifying the candidate metering device as being out of calibration based on the difference value includes calculating an additional difference value between the power measurement value of the candidate metering device and an additional comparison value of the plurality of comparison values, and determining if the difference value and the additional difference value are positive or negative; and identifying an increased likelihood of the candidate metering device being out of calibration in response to determining the difference value and the additional difference value are both positive or both negative.