Potentially inaccurate CT clamp readings

[andrewmoore]

Hi all,

I've noticed recently that the grid CT clamp reading for the for my FoxESS H1-5.0-E inverter differ from other sources.

For example, on Friday these were the reported grid import numbers:

• FoxESS Inverter: 13.4 kWh
• Electricity Meter: 14.52 kWh
• myenergi Harvi: 15.48 kWh

Now, it's hard to pinpoint which device is trustworthy because there is such a spread. I'd understand if there was a difference of 2-3% between readings but a difference of 15% seems excessive.

Yellow = myenergi | Blue = Meter | Red = Inverter

Anyone able to offer insight as to possible causes? Can the FoxESS clamp be relied on as the source of truth?

Thanks in advance.

[Dave Foster]

Your smart meter is what you get billed on so that is the source of truth, it’s also highly accurate and takes into account power factor.
CT clamps as a rule are between 2 & 5% accurate, but their accuracy is better at higher loads and worse at lower loads, positioning (close proximity to other CT clamps) and cable length will also play a part.

Whilst the Fox CT won’t be precisely accurate it should be relative consistent in its accuracy so once you know the scale of the difference to your smart meter it should stay very close to that.