Voltage Transformers (VTs) provide a reference voltage and phase information to IoTaWatt so that real power (what is recorded by the meter) can be measured. These are UL Listed low-voltage AC wall transformers. They are used exclusively as a reference and do not power the IoTaWatt.
The IoTaWatt W14-NA comes complete with the necessary voltage transformer for standard 120V service in North America.
I'm currently working to secure a source of compliant transformers for 230V Euro plug.
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