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Although possible via adding kwh data in emoncms. I am, with Robin Emley's help actually working on improving this right now. To actually calculate the accumulated kwh per channel on the emontx itself, with the value stored in eeprom so that it always accumulates like a standard billing meter. It could then be possible set the initial value to the value of your billing meter when you install the emontx and so have a way to check exactly how close they are. 

Ah that sounds good although might I be so bold as to suggest being able to set a date/time against the initial reading rather than from install?  If not folk with data would have to start from scratch.

In the processing there is an accumulator.  What is that?  There is no documentation for it.

I am with you on this borpin.  I have a new installation and would like to check against the house meter but cannot find a way to check an accumulated usage over a defined period.  Trystans idea sounds very good.  But if it's stored in the emontx itself could the values be manipulated to allow for corrections and how could a calibration adjustment be added as corrections seem to be usually done in the basestation.

Trystan, you may know this already but in case not, if you plan doing any serious work with EEPROM, I use and recommend using the AVR libraries directly, rather than what little the Arduino library offers (or did last time I looked).

The EEMEM attribute makes managing your EEPROM space a breeze, and eeprom_read/write/update_block() makes it trivial to keep arbitrary C structures out there.  The _update_ series of routines only write if your newly supplied values are different to what's there now, meaning you can even be fairly extravagant with your "writes" safe in the knowledge they'll only happen if they need to.

http://www.nongnu.org/avr-libc/user-manual/group__avr__eeprom.html

I would suggest any calibration is done on the Input to feed part of Emoncms.