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Metering accuracy FAQs
Metering accuracy refers to precisely measuring and reporting energy usage.
Table of contents:
Metering accuracy is an important factor in moderate to critical electrical applications. Accuracy refers to a device's ability to measure and report compared to the actual value the device sees. This is similar to a musician’s ability to playing a song “on key.”
Accuracy is often given as a percentage (similar to percent error) over an operating range. For example, +/- 0.2% accuracy means when the true value is 208V, the meter may report anything between 203.84V and 212.16V.
Metering accuracy and sampling rate are unrelated.
A higher sampling rate does not necessarily mean better accuracy. A good analogy to illustrate this is the concert video example.
Metering accuracy refers to a device’s ability to report the actual value it is measuring, which is similar to a musician’s ability to playing a song “on key.” A musician with lower accuracy would play “off key.”
Imagine this musician is giving a concert; they are being recorded in HD video and by an old VHS camcorder. These two cameras represent higher and lower resolution (sampling rate). The musician’s ability to play accurately is unrelated to the resolution of the video footage. Both aspects play into the overall watchability of the concert recording.
The American National Standards Institute (ANSI) has been defining the performance criteria of electricity meters for more than 100 years. Everything from dimensions and form factor designations to electrical ratings (including accuracy) is covered by this accreditation board.
This gives manufacturers and users a common terminology to describe an electric meter's performance, which increases manufacturer transparency and reduces misunderstandings when selecting the correct device for a given application.
ANSI C12.20 refers to the standard for accuracy and performance.
As of 2015, the standard lists a battery of 44 separate tests that must be passed to claim compliance and ensure the meter performs across different applications.
These tests consider variety of inputs (changes in load, power factor, temperature, harmonic distortion, etc.) to ensure a device will perform under different operating conditions. This provides assurance a metering device will measure accurately whether it's being used for an apartment building in Alaska or a manufacturing facility in Florida.
ANSI has defined the following accuracy classes:
Class 0.5 — error less than +/- 0.5% of true value;
Class 0.1 —error less than +/- 0.1% of true value;
Applications where ANSI accuracy compliance is important include:
Most applications where ANSI accuracy and compliance matter involve a monetary or financial tie-in, so a high degree of accuracy is required. You may hear the requirement of “revenue grade metering” or “revenue accurate” metering, referring to the fact that money is changing hands based on the metering results.
Other applications requiring a high degree of accuracy include cases where the meters are being used to monitor sensitive loads or processes. In critical power applications, where power interruptions can have devastating consequences, an overloaded circuit may result in a tripped breaker and costly downtime and potential equipment damage. In these circumstances, correct metering accuracy is critical.
Where would non-revenue grade meters be used?
While they are not appropriate for billing purposes, meters that are not revenue grade provide valuable information for understanding and managing energy usage to make informed decisions.
Applications include:
Using a mix of non-revenue grade meters alongside revenue grade or ANSI compliant meters can be a cost-effective way to add additional metering to your electrical system.
Metering manufacturers may make accuracy claims with by providing a percentage for particular metering parameter (e.g., +/- 0.05% energy accuracy) or by using the generic descriptor "revenue metering accurate."
This is generally an indication they are not fully compliant with the ANSI standard. These devices may be capable under specific circumstances but may not be designed for performance across the entire range of operating conditions.
These devices may still provide valuable data and be sufficient and appropriate for certain applications but buyers should exercise caution if they wish to use these meters as a reference for certain reporting and billing applications. In some areas, billing with non-ANSI-compliant meters is not permitted.
