Current transformers, when used with either the emonTx or emonPi and a voltage reference to give phase information, are capable of detecting the direction of power flow.
Provided that you can measure the correct quantities, you can do maths on them - so (say) you measure everything except your house usage, then that’s what is left after you added all the others together, observing the correct sign.
It is indeed both overkill and dearer - unless you might have need at some time in the future for two more channels. An emonPi is a two-channel emonTx and a Raspberry Pi in the same box, with an LCD display thrown in. An emonBase is a Raspberry Pi with the radio receiver but no “emon” power measuring circuitry.
What you can do is have an emonTx, forget the radio in it, and connect it serially to a Raspberry Pi (an RPi not even with the radio that makes it an emonBase). This is exactly how the “emon” card talks inside the emonPi. You miss out on the aluminium case and the LCD display.
The minus side of that is, once you have used the serial connection, no further expansion is possible, so if (say) you want a few emonTHs around the house, you’ve got to backtrack, fit the RFM2Pi radio module to your RPi (thus making it an emonBase), turn on the emonTx’s radio and then you can have it, the emonTHs and another emonTx, and… all talking to it by radio.
I’d strongly advise you to buy the FTDI programmer, as you’ll probably need it at some point to reload or configure your emonTx, even though you won’t need to rewrite the software in it.
So it looks as if your shopping list is 4 c.t’s, one a.c adapter, one emonTx, one programmer and a few bits of wire for the serial connection to your RPi.
Sorry to be a pain Robert, on the page you linked to it shows 3 cables between the PI and The EmonTX, the text references a 5v power rail however both the pi and the tx in the picture have there own power…would I actually need to connect the 5v or is that providing power to another part of the system?
I’d be inclined to not use the PI’s 5 V to power the emonTx, the internal 3.3 V from the a.c adapter is likely to be much cleaner, so you’ll be less troubled by noise creating a phantom standing current in the inputs.
I’m going to have a play with this setup when I get the chance. My feeling is that an RpiZeroW with emonhub is a better solution than the ESP8266 - I’ve had such poor experience with Wi-Fi on ESP8266 recently. It would also then give you buffering of data etc for about the same price.
Of course, if it works on a ZeroW it would work on any Pi. I have previously used an OrangePi but I felt this was under powered and the Wi-Fi on that even worse!