Welcome, Rhodri to the OEM forum.
I can’t help you with the WiFi problem - you need an emonCMS expert for that.
Your emonTx - which sketch and library is it using? Did you specifically order it for battery operation?
If you’re running on batteries, you need the older sketch and the old emonLib, which samples the currents for 300 ms every 10 s (and goes to sleep in between, thus saving battery power). If you ordered it for battery operation, the shop has slipped up so email them, mentioning this thread. Your screenshot indicates you might be using the newer sketch that uses the continuous monitoring (“CM”) library, which not only will use batteries faster, it also XORs each data byte with 0x55 to avoid sending long streams of ‘0’ bits over the radio, which the receiver in your emonBase can struggle with. It seems highly likely that the version of emonHub that you have doesn’t do the XOR operation to reverse the process.
(Try adding “whitening = 1” to the Node15 definition in emonhub.conf:
nodename = emontx3cm15
names = MSG, Vrms, P1, P2, P3, P4, E1, E2, E3, E4, T1, T2, T3, pulse
datacodes = L,h,h,h,h,h,L,L,L,L,h,h,h,L
scales = 1,0.01,1,1,1,1,1,1,1,1,0.01,0.01,0.01,1
units = n,V,W,W,W,W,Wh,Wh,Wh,Wh,C,C,C,p
whitening = 1
If your emonHub is capable, that should restore sensible numbers - but only with the “CM” sketch - the discrete sample sketch doesn’t do “data whitening”.)
If you’re staying with battery operation, I think you need a programmer to reload the sketch in your emonTx. The one you want is here: https://github.com/openenergymonitor/emontx3/tree/master/firmware
(If you don’t have the Arduino IDE to compile and load it, all you need to know is here: Learn→Electricity Monitoring→Using the Arduino IDE