This two-way assurance rests upon a short chain of calibrations anchored at the National Institute of Standards and Technology (NIST) where the ultimate power meter lives. The accuracy of every watt-hour meter in the country ultimately is traceable to the Electricity Division of NIST's Electronics and Electrical Engineering Laboratory. Most watt-hour meters are electromechanical devices in which a tiny portion of the electrical power going through it is converted into the mechanical clock-like motions that move the meter's dials. Just as clocks can be fast or slow, so too can watt-hour meters be off. That is why state public utility commissions (PUCs) own and maintain standard watt-hour meters with which they can certify the accuracy of mass-manufactured meters. Meters pass when they produce the same power reading as the standard meter when the same amount of current passes through them.

NIST provides the ultimate basis for these measurements because the standard meters of the PUCs go through periodic calibrations at NIST in which the amount of electricity going through a meter can be more accurately and confidently measured than anywhere else in the country. Once the standard watt-hour meters pass muster at NIST, they can serve as genuine gatekeepers for the much larger population of residential and business watt-hour meters.