Sensing methods

CT (Current Transformer) clamp

An inductive sensor (CT clamp) is positioned on the mains wire where it enters an existing electricity company supplied meter outside the house. The CT sensor does not penetrate the wire or make any direct electrical connection to the mains. The CT method can be used to monitor just one circuit within the house by positioning the clamp on that circuit within the household breaker box / fuseboard.

Accuracy depends on several factors:
  • Quality of the CT clamp provided
  • Placement of the CT proximity clamp
  • Fluctuation and range of the mains supply voltage
  • Ability of the monitoring device to interpret and display data correctly

Efergy: "efergy energy monitors are accurate to within ±10%, at least. In most cases they are accurate to within 5%. Most of this variation is due to fluctuations in your electricity supply"

A direct interface to your electricity meter is required to get a 100% (theoretically) exact measurement of the electricity you are being charged for. In practice many other factors may influence accuracy.

Pulse / Optical monitoring

Many smart meters have pulse or optical output. A simple light sensor can be used to monitor an optical pulse output. Different meters have different pulse/kwh ratios. Some smart meters provide pulse output for both active and reactive power. GeekZone has photos of a few smart meters (with optical output) deployed in New Zealand.

Energy monitors with optical sensors in NZ

Current Cost Envi-R plans to release an optical reader in 2012. SmartNow state: "The [Current Cost] EnviR can provide 100% accurate billing information if you have a pulse meter and using the forthcoming Optical Readers".

The Watts Clever Wireless Energy Monitor for Smart Meters advertises accuracy +/- 5%.

Combination sensor - Spinning Disc Sensor / Optical output

The Blue Line Innovations PowerCost monitor uses a combination sensor to monitor either the optical output common on smart meters or the spinning disc common on older style electricity meters. Pages 4 and 8 of this user manual for the BL 28000 show the supported meter types and sensor configuration. A post on the user forum indicates a multiplier is used to adjust for different pulse / disk frequencies. The compatibility page for the current model has a list of known supported meter types. About accuracy Blue Line say: "Due to when your meter is read by your utility and due to other utility charges, the "total" readings on your display will not exactly match what you see on your electricity bill. It is accurate within five percent of the actual amount". This product is also available as the Black And Decker Power Monitor. These products are not available in NZ.

HAN (Home Area Network) interface

Next generation smart meters have a built in wireless HAN network chip. HAN monitoring devices can communicate with the meter using this network to get very accurate measurements. Many in meters in NZ have HAN capability but no HAN module installed. 
Very few smart meters with an operational HAN device have been deployed in New Zealand. Some HAN devices were deployed during field trials. 

Non-intrusive load monitoring

Non-intrusive load monitoring is a data processing / analysis method and is not dependent on a particular sensing device.

In simplest form - if your hot water heater has a 3000 watt element and your display or graph output shows an increase of around 3000 watts you can be sure this indicates your water heating just turned on. A relatively simple computer program is able to perform this analysis automatically. Depending on the resolution of the data, individual appliances and operation cycles can be identified with some accuracy.

In advanced form - multiple samples per second of two line factors are required. When analysed by a computer these high resolution waveform samples can identify each individual appliance with a high degree of accuracy. Appliance fault conditions corresponding to particular waveforms can be identified.

More information about non-intrusive load monitoring.


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