@Anticimex I do agree. It CAN be used, but it is just a bad choice as the ethernet libraries do not leave much space for much else.
ENC28J60 in that aspect is worse than the W5100.
I tried it more like a proof of concept and it does work pretty well just passing data, but I put everything on an ESP8266. OK, WiFi rather than Ethernet but thats OK for me

@dynamite yes i do not see a problem in measuring the amplitude. I am not sure if, when measuring current through a CT, the frequency will change, as that is just the grid frequency, but probably you were referring to its use in measuring gas flow.

My idea to use a dedicated attiny is simply so the ESP will be freed of the burden of calculation/measuring, as this will be a rather continuous process. The code will probably fit an attiny13, but I have never tried i2c on an attiny13, but I have on an attiny85, so i am pretty sure it will work on the 25 and 45 as well. The 13 I still have to try

@dynamite indeed, that is why i was looking at yr code. On the other hand, if the frequency is known and you know the amplitude, you can make a calculation.... but then again, in order to know the amplitude you need a series of measurements.
Thats why i was thinking to let a dedicated attiny do that: all it needs to do is to measure and put the result in its I2C registers. The esp8266 then can do a hoist of other things and just pull the powerparameters when needed

@Yveaux yes, As a matter of fact I looked into that one, I think they can be had for 60cts, but the 'problem' is they need a galvanic connection to the grid and to the esp as well. I want it to be completely isolated from the grid

@dpcr thanks, indeed, I am using a CT as well, adding a dc bias, still balancing between just measuring the amplitude of the sinus, or measuring all along the sinus. I think i will have an attiny 25 or ideally an attiny13 do this as a dedicated I2C slave and have that read by an ESP8266. Ofcourse there is the sonoff Touch, but thats a tadd too much to put on a large number of devices.

@dpcr Thanks, yes i read how you did it with locating the sweet spot, but I was thrown off by the 'no magnet' statement.
'connecting' my meter is also still in the cards, no idea if it has a magnet or not (in which case i may use a Hall sensor), otherwise i may try the magnetometer

@dynamite thanks, i know pretty well how the magnetometer works, but I guess i was thrown off by the statement 'no magnet', but I guess it must be weak magnetization of the various parts then

@dpcr very interesting, I may use some of your sine measuring approach in a powermeter i make.

Nevertheless, i have a question: you say you have no magnet in yr meter, so what is it the magnometer is actually picking up?

@jpaulin OK, thanks. Just seemed like a rather long antenna in the picture.
I am using this antenna, with very good results. Better than the 1/4 wavelength whip

@jpaulin Thanks, that is good to know. I have the RFM69 HW running with a W5100/Promini. W5100 module needed a bit of a hardware tweak for SPI, but works well. Now I probably would have opted for the 5500 as well.
I just use it to communicate between two properties at some 500 meter distance, most sensors I use are attached to ESP8266

Seems like a rather long Antenna on yr module. What frequency are you on?