# How accurate is current sensing in home using current transformers

There a lot of solutions in the market to mesure your house electricity consumption, and the number is raising every day. I realize that plenty of them use current transformer clamps:

In today's homes, current is not sinusoidal thanks to electronics devices, and if I remember correctly from my engineering classes, this currents can be represented as a infinite sum of sine waves of different frequencies (Fourier), the most chaotic the current, higher frequency components you got, and high frequency electromagnetic fields are going to saturate the ferrite in the clamp compromising the accuracy of the measurement.

How good are this systems? Whats the minimum power factor you can have and still consider the measurement good enough?.

• Non sinusoidal waveform is not a problem for the transformers, its a problem for the equipment that reads the signal after the transformer. You can do it the same crappy non TrueRMS way that cheap power meters do with a shunt too. Aug 29, 2016 at 11:50
• Yes but, non sinusoidal waves creates fields with frequencies different from 50/60hz that do not induce the same amount of current the sensor is calibrated to read. Aug 29, 2016 at 11:57
• These energies are not lost, they just transfer to a different amount, which is usually all summed up in the accuracy value of the devices datasheet. Usually the current transformers are fine up to several hundreds of Hz, which in itself is often good for less than 1% of error in reading. Aug 29, 2016 at 12:00
• On the contrary, high frequencies are going to saturate the ferrite core less than low frequencies. Aug 29, 2016 at 13:42
• Have a look at openenergymonitor.org where you should find plenty of details on hardware and practical measurement. Aug 29, 2016 at 22:02

The "Home Energy Monitor" you pictured costs US$240. So to be 'good enough' it must save you over$240 in electricity charges. How accurate does it need to be to do that?

Very low power factor with high harmonic content mostly occurs in devices that rectify the mains and are running at low power (eg. TV on standby) as they only draw current during peaks of the AC voltage wave. These devices contribute so little to your total power usage that measurement accuracy isn't particularly important.

high frequency electromagnetic fields are going to saturate the ferrite in the clamp compromising the accuracy of the measurement.

No, saturation is only caused by excessive peak current. High current devices generally have relatively high power factors and/or sinusoidal current draw, so this shouldn't be a problem. At very high frequencies the Ferrite will suffer eddy current loss, but such frequencies should have very little energy. Also the coil will have a resonant frequency due to inter-wire capacitance, but again this should be at a very high frequency and so have minimal effect on accuracy.

To measure true power the energy monitor must multiply instantaneous current by instantaneous voltage, then average the results over at least one cycle. It may do this by digitizing the voltage and current at a high sampling rate, then calculating the result in software. Current (and voltage!) measurement accuracy will be reduced as harmonics approach the Nyquist frequency (half the sampling rate), and by the bandwidth of the conditioning amplifier and input filter.

In the end the only thing that matters is the reading on your power company's meter. If your Home Energy Monitor helps you to reduce that figure then it is doing the job, even if it doesn't read low power factors very accurately. TED monitors are purportedly calibrated to 2% accuracy. How well they can maintain that accuracy with low power factor non-sinusoidal waveforms is not specified, but it probably isn't important.