Accuracy at low power?


I have a system based on an arduino with EmonTX shield. A while ago I was measuring a group which only had a microwave and nothing else. My system told me it was using 12 Watt in standby, which seemed rather high.

I then also measured with a Cresta RCE-1106, and this showed me 3 W.

(I had calibrated my system with a 70W lightbulb. In that case it gave exactly the same result as my Cresta.)

Could it be that the system is less accurate at lower power levels?


Robert Wall's picture

Re: Accuracy at low power?

Without knowing which CT and burden resistor you are using, it is impossible to know what "lower" means. All CTs are inherently inaccurate at low currents (it follows from considering the core losses), and unless you have a "revenue" grade one, the majority are only specified down to around 5% of maximum current. That is the CT alone, and does not consider quantization errors in the ADC, which start to become significant when the amplitude of the input falls below about 10 counts (i.e. about 2% of maximum current.

dBC's picture

Re: Accuracy at low power?

If your microwave is anything like mine, it could be a phase error also.  My microwave's apparent power in standby is about 90W  (or 90VA strictly speaking),   but it's actual power is only 3W.  Does your Cresta tell you the power factor when it's displaying 3W, or if not, can it tell you how much current is flowing?

When you've got that much reactive power flowing, and very little real power, a relatively small phase error can create a massive error in the real power.  

hneel's picture

Re: Accuracy at low power?

The sensor is the one from this shop

I have a 9V AC adapter connected to the shield. This is one that I had lying around in my home. This one is not mentioned as an example on this site.

Here is my arduino sketch for the shield:


Fixed links - BT




hneel's picture

Re: Accuracy at low power?

I just took another look with the Cresta. Here are the results:

211 Volts / 50Hz.

0.03 Ampere / pf = 0.71

4 Watt.


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